feat(v3): add "Add to playlist" to a song's ⋮ More menu (#625)

* feat(v3): add "Add to playlist" to a song's ⋮ More menu

You could only add a song to a playlist via select-mode (checkbox → batch bar).
Add an "Add to playlist" row to each song card's ⋮ overflow menu that targets
that one song, reusing the same picker (pick a listed number or type a new name
to create the playlist).

The select-mode batch flow and the single-song menu now share one extracted
`addFilenamesToPlaylist(filenames)` helper; the menu is `openCardMenu`, shared by
grid cards and tree rows, so both views get it. Tests:
tests/js/v3_add_to_playlist_menu.test.js.

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

* fix(v3): don't clear the batch selection when the playlist picker is cancelled

The extract-helper refactor made batchAddToPlaylist() call finishBatch()
unconditionally, so cancelling (or a failed create) cleared the multi-select
and reloaded the grid — a regression from the original early-return-on-cancel
behaviour. addFilenamesToPlaylist() already returns null on cancel/failure;
gate finishBatch() on a truthy playlist id so the selection is preserved for
a retry. Adds a regression assertion.

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
+- **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).
 - **Folder Library — a bundled core plugin (`plugins/folder_library/`) that browses the DLC library by its on-disk folder tree.** Surfaces top-level folders → subfolders → songs (root-level songs land in `(Unsorted)`), with in-app folder management (create / rename / delete nested folders), song moves via dialog or drag-and-drop, and sort/filter that mirrors the host library's filter state. Wired into both the classic (v2) library toolbar and the v3 Songs page as a third **Folders** view alongside grid/tree; the plugin's `screen.js` is loaded once by the host and reused (idempotent IIFEs). Supersedes the former standalone "Folder Organizer" community plugin (removed from the README list). Backend (`routes.py`) registers `/api/plugins/folder_library/{tree,folder/create,folder/rename,folder/delete,song/move}`; **all filesystem mutations are confined to `DLC_DIR` and validated against path traversal** (per-segment name validation plus a resolved-containment check on `song/move`), and folder deletion relocates every song — de-duplicating colliding names — so a name clash never destroys a song. A two-level cache keeps re-opening folders fast. Tests: `tests/plugins/folder_library/test_routes.py` (path-safety helpers + move-traversal and delete-no-data-loss end-to-end).