feat(harness): agent-drivable dev harness and CI that gates

A coding agent could develop this repo's Go packages and could not
develop the application: every path to running YellowJacket ended in a
blocking GTK window, so 265 bound methods, 46 events, 33 component
directories and 13 stores had exactly one form of verification
available — `tsc --noEmit`.

The unlock is that `wails dev`'s dev server on :34115 serves the real
frontend with the real generated bindings against the same Go backend a
desktop window attaches to, so a plain Chromium under Xvfb gets a fully
functional app. Four test tiers now exist, cheapest first:

- `make ui-test` — 313 Vitest tests in a real browser in ~2 s, no app,
  no backend, no display. Works because `frontend/wailsjs/` is a pure
  passthrough to `window.go`/`window.runtime`, so faking just those two
  globals runs the real bindings and the real store code.
- `make test` — services in-process, asserting on the payload the
  frontend would receive, via a new `events.Emit` wrapper.
- `make dev-headless` + `playwright-cli` — the real app, driven
  interactively, with an event bridge on `window.__yjEvents` and a
  dev-only control surface at `/__test/`.
- `make e2e` — 19 of those flows frozen as Playwright specs.

`events.Emit(ctx, …)` replaces all 35 direct `runtime.EventsEmit` call
sites: wails' `getEvents` `log.Fatalf`s on any context without its
runtime, so those paths could not run under test and a background
worker could take the app down. Four packages had each hand-rolled the
same guard; nine more guarded on `ctx != nil`, which does not help.
`TestNoDirectRuntimeEmits` fails the build on a new one.

Fixtures are generated, not committed (`make testdata`), and seeds are
built by *running the app* — never by hand-writing config and DB rows,
which would be a second description of a valid YJ_HOME.

`.gitea/workflows/ci.yml` is the first workflow here that tests
anything; the other three only package, so `gitea_ci` reported only
packaging jobs and misled anyone asking whether a push was healthy.
Both jobs were prototyped to green in a bare ubuntu:24.04 container
before the YAML was written, which immediately caught `make lint`
linting three configurations that nothing builds: all three passes
omitted `webkit2_41`, so wails resolved webkit2gtk-4.0 — which Arch
still ships and Ubuntu 24.04 dropped.

Operational instructions live in `.pi/skills/yellowjacket-dev/`,
measured discoveries in `.planning/NOTES.md`, and architecture in
`CLAUDE.md` — split by tense, not by topic, because a topical split
gives every new fact two plausible homes. `make skill-check` fails a
commit if the skill cites a make target that does not exist.

frontend/src/components/audio-player/controls/player-controls.ts

@@ -119,19 +119,29 @@ export class PlayerControls extends LitElement {
 
     return html`
       <div id="player-control-buttons">
-        <button class=${shuffleClass} @click=${this.handleShuffleClick}>
+        <button
+          class=${shuffleClass}
+          aria-label="Shuffle"
+          aria-pressed=${this.shuffleMode}
+          @click=${this.handleShuffleClick}
+        >
           <wa-icon name="shuffle"></wa-icon>
         </button>
-        <button @click=${this.handlePreviousClick}>
+        <button aria-label="Previous track" @click=${this.handlePreviousClick}>
           <wa-icon name="backward-step"></wa-icon>
         </button>
-        <button @click="${playOrPauseHandler}">
+        <button aria-label=${this.player.isPlaying ? 'Pause' : 'Play'} @click="${playOrPauseHandler}">
           <wa-icon name=${playOrPauseIcon}></wa-icon>
         </button>
-        <button @click=${this.handleNextClick}>
+        <button aria-label="Next track" @click=${this.handleNextClick}>
           <wa-icon name="forward-step"></wa-icon>
         </button>
-        <button class=${repeatClasses} @click=${this.handleRepeatClick}>
+        <button
+          class=${repeatClasses}
+          aria-label=${`Repeat: ${repeatMode}`}
+          aria-pressed=${repeatMode !== 'off'}
+          @click=${this.handleRepeatClick}
+        >
           <wa-icon name="repeat"></wa-icon>
         </button>
       </div>

frontend/src/components/audio-player/seekbar/seek-bar.ts

@@ -155,8 +155,9 @@ export class SeekBar extends LitElement {
 
     return html`
       <div id="seek-bar-container">
-        <small>${elapsedTime}</small>
+        <small data-testid="elapsed-time">${elapsedTime}</small>
         <wa-slider
+          aria-label="Seek"
           .value="${this.seekValue}"
           max="${this.trackLength}"
           ?with-tooltip="${this.hasTrack}"
@@ -165,7 +166,7 @@ export class SeekBar extends LitElement {
           @change="${this.handleChange}"
           @input="${this.handleInput}"
         ></wa-slider>
-        <small>${remainingTime}</small>
+        <small data-testid="remaining-time">${remainingTime}</small>
       </div>
     `;
   }

frontend/src/components/cover-grid/scroll-manager.ts

@@ -243,6 +243,12 @@ export class ScrollManager {
     ): void {
         // Guard against stacked observers.
         this.resizeObserver?.disconnect();
+
+        // An empty library renders no scroll container, so the caller's
+        // query returns undefined and observe() throws — asynchronously,
+        // out of loadAlbums, where nothing catches it.
+        if (!container) return;
+
         this.currentColumnCount =
             this.getColumnCount(container);
 

frontend/src/components/now-playing/now-playing.ts

@@ -339,6 +339,7 @@ export class NowPlaying extends LitElement {
           <div class="track-info">
             <span
               class="track-title ${titleScrolling ? 'will-scroll' : ''} ${this.titleScrolling ? 'scrolling' : ''}"
+              data-testid="now-playing-title"
               @mouseenter=${this.handleTitleMouseEnter}
               @mouseleave=${this.handleTitleMouseLeave}
               @transitionend=${() => this.onScrollCycleEnd('title')}
@@ -347,6 +348,7 @@ export class NowPlaying extends LitElement {
             </span>
             <span
               class="track-artist ${artistScrolling ? 'will-scroll' : ''} ${this.artistScrolling ? 'scrolling' : ''}"
+              data-testid="now-playing-artist"
               @mouseenter=${this.handleArtistMouseEnter}
               @mouseleave=${this.handleArtistMouseLeave}
               @transitionend=${() => this.onScrollCycleEnd('artist')}

frontend/src/components/queue-panel/queue-panel.ts

@@ -1412,6 +1412,8 @@ export class QueuePanel
                     'drop-after': showAfter,
                 })}
                 data-index=${index}
+                data-testid="queue-row"
+                data-file-path=${track.filePath}
                 draggable="true"
             >
                 <span class="track-position">

frontend/src/components/sidebar/app-sidebar.ts

@@ -215,6 +215,10 @@ export class AppSidebar extends LitElement {
             return html`
                         <li
                             class=${classes}
+                            data-testid="nav-${item.id}"
+                            aria-current=${this.activeView === item.id
+                    ? 'page'
+                    : 'false'}
                             @click=${() =>
                     this.navigate(item.id)}
                             @dragover=${(e: DragEvent) =>

frontend/src/components/track-list/track-list.ts

@@ -1740,6 +1740,8 @@ export class TrackList extends LitElement implements SelectionHost, ContextMenuH
         })}
         draggable="true"
         data-index=${index}
+        data-testid="track-row"
+        data-file-path=${track.FilePath}
       >
         <div
           class=${classMap({