feat(v3 library): A–Z fast-scroll jump rail on the Songs grid (#634)

* feat(v3 library): A–Z fast-scroll jump rail on the Songs grid

Adds a vertical letter rail (Plex/Radarr/iOS-contacts pattern) pinned to the
right edge next to the scrollbar so you can jump the library to a starting
letter — tap, drag-to-scrub with a live letter bubble, or arrow-key between
letters. The classic (v2) tree already had letter selection; this brings the
new v3 grid to parity (it was the gap behind the "alphabetical scroll
selection next to the scrollbar" idea).

It shows ONLY for the grid view + alphabetical (artist/title) sorts, and only
offers letters present in the current sort AND filter set, so a tap always
lands on a real card (absent letters are dimmed + non-interactive). The grid
is forward-only, server-paged infinite scroll with no virtualization, so a
jump pages through to the target card then scrolls to it; a token guards
overlapping jumps (drag) so the newest wins. A keyset-seek + virtualized
window is the scaling follow-up for very large libraries.

Backend: /api/library/stats gains an optional `sort` param and an additive
`sort_letters` map — songs-per-first-letter of the ACTIVE sort column (artist
or title), filter-synced — so the rail's present-letters match the grid's real
order. The legacy `letters` (distinct-artist) field is unchanged, so the
dashboard + classic tree are unaffected. `sort` is dropped for providers whose
query_stats predates it (existing kwarg-filter), so third-party library
providers keep working (rail simply falls back / hides).

Frontend: static/v3/songs.js (refreshRail / jumpToLetter / pointer-drag +
keyboard, cards tagged data-letter), static/v3/v3.css (.v3-azrail + bubble).

Tests: tests/test_library_filters.py (sort_letters artist/title, song-vs-
distinct-artist counting), tests/test_library_providers.py (sort forwarded),
tests/js/v3_az_rail.test.js (gating, data-letter, load-through, drag/keys).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QbexxfTt8q2tAn436MqGWF

* fix(v3 library): harden A–Z jump rail (review P2/P3)

Addresses the PR #634 review findings (manual + Codex):

P2 correctness
- refreshRail prefers the active-sort `sort_letters`; falls back to the
  artist-based `letters` only on an artist sort, and hides the rail on a
  title sort when a legacy provider returns none (was mislabeling letters).
- reload() bumps `_jumpToken` so an in-flight letter jump can't scroll a
  grid that's being rebuilt from page 0.
- songBucket no longer trims, matching the server SQL + grid ORDER BY raw
  first-char bucketing (a leading-space title now buckets under '#' on both
  sides).

P3 polish
- Paging guard is total-derived (ceil(total/PAGE_SIZE)+2) instead of a
  magic 4000, keeping large libraries reachable while still bounded.
- Roving tabindex: only the first present letter is tabbable; arrow keys
  move it. Removes up to 27 page tab stops.
- `sort_letters` is computed only when the caller opts in
  (want_sort_letters / route `sort_letters=1`); the dashboard + v2 tree
  skip the extra GROUP BY. Added sort + want_sort_letters to the optional
  provider-kwargs so non-introspectable legacy providers drop them.
- _railToken supersedes stale refreshRail responses; hide the rail when no
  letters are present instead of rendering disabled buttons.

Tests updated accordingly (v3_az_rail.test.js, test_library_filters.py).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: byrongamatos <xasiklas@gmail.com>

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- **A–Z fast-scroll rail on the v3 Songs grid.** A vertical letter rail (Plex/Radarr/iOS-contacts pattern) pinned to the right edge next to the scrollbar lets you jump the library to a starting letter — tap a letter, drag to scrub with a live letter bubble, or arrow-key between letters. It shows **only** for the grid view + alphabetical (artist/title) sorts, and only offers letters actually present in the current sort **and filter set**, so a tap always lands on a real card (absent letters are dimmed + non-interactive). Because the grid is forward-only, server-paged infinite scroll, a jump pages through to the target card and scrolls to it (a newer jump supersedes an in-flight one); a keyset-seek + virtualized window is the noted scaling follow-up for very large libraries. Backend: `/api/library/stats` now accepts `sort` and returns an additive `sort_letters` map (songs-per-first-letter of the active sort column — artist or title), filter-synced; the legacy `letters` (distinct-artist) field is unchanged for the dashboard + classic tree. Frontend: `static/v3/songs.js` (`refreshRail`/`jumpToLetter`, cards tagged with `data-letter`), `static/v3/v3.css` (`.v3-azrail`). The classic (v2) tree already had letter selection; this brings the new grid to parity. Tests: `tests/test_library_filters.py` (sort_letters artist/title + song-vs-artist counting), `tests/test_library_providers.py` (sort forwarded to providers), `tests/js/v3_az_rail.test.js`.
 - **Playlists get content-dependent covers + custom art.** Playlist cards were a tiny `🎵` emoji on an empty square. Now a playlist's cover reflects its contents: **empty → the icon**, **a few songs → the first song's album art**, **4+ songs → a 2×2 art mosaic**. You can also **upload a custom cover** (a "Cover" button in the playlist detail view → image picker; "Remove cover" reverts to the content view). `MetadataDB.list_playlists()` now returns each playlist's first few song `art_urls`; `GET /api/playlists` and `GET /api/playlists/{id}` add `cover_url` when a custom cover exists. New routes `POST` / `GET` / `DELETE /api/playlists/{id}/cover` store a small PNG thumbnail under `CONFIG_DIR/playlist_covers/` (PIL-converted, like song-art upload); the cover is removed with the playlist. Frontend: `playlistCoverHtml(p)` in `static/v3/playlists.js`. Tests: `tests/test_playlists_api.py` (art_urls + cover roundtrip / reject-non-image / delete-cleanup), `tests/js/v3_playlist_cover.test.js`.
 - **v3 Songs: "Add to playlist" is now on each song's ⋮ "More" menu.** Previously a song could only be added to a playlist through select-mode (the checkbox → batch bar). The per-card overflow menu now has an **Add to playlist** row that targets that one song, reusing the same picker (choose a listed number or type a new name to create it). The select-mode batch flow and the single-song menu now share one extracted `addFilenamesToPlaylist(filenames)` helper in `static/v3/songs.js` (both grid and tree rows, since they share `openCardMenu`). Tests: `tests/js/v3_add_to_playlist_menu.test.js`.
 - **Resume where you left off — leaving a song now snapshots your place so an exit is recoverable, not a restart-from-zero.** Exiting the player (`showScreen()` teardown, before audio unload) writes `{song, arrangement, position, speed}` to `localStorage` (`feedBack.resumeSession`), and a non-blocking **"Resume practice"** pill offers it back on the next non-player screen (and on the next app launch). Clicking Resume re-enters the song, restores the arrangement + playback speed, and seeks to the saved position via the existing `_audioSeek` funnel; `playSong()` gains a `{ resume: {position, speed} }` option that arms a `song:ready`-consumed restore instead of the normal autostart, so the two never fight over playback. The snapshot is deliberately conservative — ignored for a song you barely started (< 3s) or had basically finished (within 5s of the end), cleared on natural song-end and once consumed, and expired after 24h. The pill is self-contained (inline-styled, body-appended, works identically in the classic and v3 shells with no Tailwind rebuild), never blocks, and a dismiss forgets the current snapshot for the session. This pairs with the Escape focus fix: now that Escape reliably leaves regardless of focus, an *accidental* exit is one tap to undo. Public surface: `window.resumeLastSession()` / `window.feedBack.resumeLastSession`. (The broader nav-state work — returning to a song after wandering into Settings → Tone Builder — is a separate, larger track; this lands the player-session slice.) Tests: `tests/browser/resume-session.spec.ts` (snapshot guards, staleness, pill show/hide/dismiss, resume consumption).