feat(v3): content-dependent playlist covers + custom art (#626)

* feat(v3): content-dependent playlist covers + custom art upload

Playlist cards were a tiny 🎵 emoji on an empty square. Now the cover reflects
the playlist's contents, and you can override it with a custom image.

Cover (in priority order):
- custom uploaded cover, else
- empty playlist  -> the icon
- a few songs     -> the first song's album art
- 4+ songs        -> a 2x2 album-art mosaic

Backend (server.py):
- MetadataDB.list_playlists() returns each playlist's first few still-present
  songs' art URLs (`art_urls`) for the content cover.
- GET /api/playlists and GET /api/playlists/{id} add `cover_url` when a custom
  cover exists.
- POST/GET/DELETE /api/playlists/{id}/cover — store a small PNG thumbnail under
  CONFIG_DIR/playlist_covers/ (PIL-converted, mirroring song-art upload); the
  cover is deleted with the playlist. Cover mutators added to _MUTATING_ROUTES.

Frontend (static/v3/playlists.js): playlistCoverHtml(p) renders the rules above;
the playlist detail view gets "Cover" (pick an image) + "Remove cover".

Tests: tests/test_playlists_api.py (art_urls + cover roundtrip / reject-non-image
/ delete-removes-cover — 11 pass) and tests/js/v3_playlist_cover.test.js.

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

* fix(playlists): 400 (not 500) on non-string cover image + bust same-second cover cache

Two review follow-ups on the playlist-cover endpoints:

- POST /cover did `if "," in b64` before any type check, so a non-string
  image (e.g. {"image": 123} / null) raised TypeError -> 500. Guard with
  isinstance (mirrors the avatar/song-art upload) for a clean 400. +regression
  test covering number/null/object/list.

- The cover URL busted only on int(st_mtime) (1s granularity) and GET /cover
  sent no cache headers, so a same-second replace/remove/re-upload could serve
  a stale image. Use st_mtime_ns in the cache-bust token and add the shared
  no-cache header (_ART_CACHE_HEADERS), matching song art.

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
+- **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).
 - **Optional "Ask before leaving a song" confirm (Gameplay tab, default OFF).** A new client-only toggle (`confirmExitSong` in `localStorage`, in the v3 Gameplay settings + the Gameplay "Reset" set) for players who want a guard against an accidental exit. **Off by default — Escape leaves instantly, zero change for everyone else.** When on, a *user-initiated* exit (the player-scope Escape shortcut, or the player's ✕) opens a small true-modal confirm instead of leaving; auto-exit on song-end and a results screen's own Close are unaffected (they call `closeCurrentSong()` directly, which stays the unguarded actual-exit). The confirm honors the team's refined asks: **opening it pauses the song** (so it isn't running or being scored behind the prompt) and **Stay resumes exactly what was paused**; **Escape = Stay** — the dialog's capture-phase handler *dismisses* it (now consistent with every other modal and the generic `_confirmDialog`'s Esc=cancel), so a second Escape returns you to the (resumed) song rather than leaving; **Space/Enter (or click) Leave** by natively activating the default-focused "Leave" button ("just get me out"). Pause/resume run through the canonical `togglePlay()` path (HTML5 + `_juceMode`), guarded so a count-in, an already-paused song, or a teardown/seek/end behind the modal can't mis-resume. It's a real modal (`role="dialog" aria-modal="true"` / `.feedBack-modal`) with **Tab trapped inside it** and a **backdrop click that also Stays**, so the Escape/Space focus carve-outs treat it as a trap and don't fire player-back / play-pause behind it. The player Escape shortcut and the v3 ✕ route through a shared `window.requestExitSong()` gate (the ✕ also becomes origin-aware, matching Escape). Tests: `tests/browser/exit-confirm.spec.ts` (default-off instant exit, confirm-on opens + stays, second-Escape stays, backdrop stays, Stay/Leave, Enter-leaves); the audio pause/resume is verified manually on web + desktop (the mock song has no backing track).