feat(player): global autoplay & auto-exit option (songs + lessons) (#558)

* fix(v3): pedal click opens the plugin's screen, not its settings

The v3 Pedalboard's settingsTarget() resolved settings-first, so a
plugin that ships both a screen and a settings panel (notably the
bundled Audio Engine) could only ever reach its settings from the
pedalboard — its actual page was unreachable.

Flip to screen-first (stompbox metaphor: step on the pedal, see the
pedal), falling back to settings when there is no screen. Keep a
settings fallback in openPluginSettings() when a declared screen
isn't mounted yet (installing/failed) so settings-bearing plugins are
never stranded on a toast. Drive the pedal aria-label off the same
target so it never promises the wrong surface. Update the unit test
contract to screen > settings > none.

Fixes #555

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

* feat(player): global autoplay & auto-exit option (songs + lessons)

Single Settings toggle (autoplayExit, default ON) that auto-starts a song
once it's ready and returns to the launching menu when it ends. Auto-exit
defers while a results/score overlay is on top (heuristic + holdAutoExit()
contract) so a scoring plugin's screen drives the exit. Player origin is now
context-aware (lessons return to the lessons screen via setReturnScreen()),
fixing lesson completion bouncing to the library.

Core-only; songs and lessons share the playSong -> highway path. Adds a
read-only window.slopsmith.autoplayExit getter + holdAutoExit()/setReturnScreen()
for plugins. Unit tests for the pure helpers (_autoplayExitEnabled,
_resolvePlayerOrigin, _resultsOverlayVisible).

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

---------

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

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- **Autoplay & auto-exit — a global "click it, it plays; finish, you're back at the menu" option (default ON).** New single Settings toggle (`autoplayExit` in `localStorage`, surfaced in both the v3 and classic settings screens; absence of the key = enabled) that closes the friction at both ends of the play loop. **Autoplay:** `playSong()` previously loaded a chart paused, requiring a Play press; a one-shot flag armed per fresh load is now consumed by the next `song:ready` (highway.js) to auto-start via the existing `togglePlay()` path (HTML5 + `_juceMode` + count-in). Arrangement switches / seeks reuse the same `song:ready` event but never arm the flag, so they don't auto-restart. **Auto-exit:** on `song:ended`, core returns to the launching menu after a short grace delay — unless a visible full-screen results/dialog overlay is on top (detected via `[role=dialog][aria-modal]` / `.fixed.inset-0` with a `getClientRects()` visibility test that works for `position:fixed`), in which case the return is deferred so that score screen's own Close button (calling `window.closeCurrentSong()`) drives the exit. A plugin can also defer explicitly via the new `window.slopsmith.holdAutoExit()` (called synchronously from its own `song:ended` handler — core's listener runs first). Both paths mean **no external plugin PR is required** for a results screen to be respected. **Context-aware destination:** the player's remembered origin (`_playerOriginScreen`) now honours any real launch screen instead of clamping to library/home/favorites, and a one-shot `window.slopsmith.setReturnScreen(id)` override lets the lessons catalog (`static/v3/lessons.js`) send a finished lesson back to the lessons screen — not the song library — even though the external tutorials plugin owns the `playSong` call. Also exposes a read-only `window.slopsmith.autoplayExit` getter for plugins. Songs and lessons share the same `playSong` → highway path, so both inherit the behaviour. Core-only (`static/app.js`, `static/v3/lessons.js`, both `index.html`s); the end-of-song score screen itself remains a plugin. Optional polish (not required — the overlay heuristic already covers it): external scoring/note-detection plugins (e.g. SlopScale) may call `holdAutoExit()` + `closeCurrentSong()` for an exact, heuristic-free handoff.
 - **"Song Editor" promoted to a first-class v3 sidebar item.** The editor
   plugin (`id: editor`) now gets its own dedicated sidebar entry — under the
   Library group, just below Songs — via the existing `PROMOTED_PLUGINS`