Add second adapter, thumbnails, a11y pass, and a real test suite

- Immich adapter (adapters/immich.js): swipe a self-hosted photo library
  over its REST API, proving the adapter contract works for remote
  sources, not just the filesystem.
- Refactor the preview contract from resolvePreviewPath() to
  streamPreview()/streamThumbnail(), so adapters can serve previews from
  anywhere (local file, proxied fetch, etc).
- macOS Quick Look-backed thumbnail cache (lib/thumbnails.js): real
  resized thumbnails, HEIC/HEIF, and video poster frames, with a hard
  timeout and graceful fallback to the original file everywhere else.
- Native "Browse..." folder picker (osascript) in Settings, with manual
  typing as the fallback on other platforms.
- Confirm-guarded "Empty trash" action -- the only place this project
  permanently deletes anything.
- Session resume: progress now survives a server restart and an
  unchanged Rescan via .swipeanything-session.json.
- Accessibility pass: live region item announcements, aria-labels on
  action buttons, focus-trapped shortcuts dialog with focus restore,
  visible focus rings, a real role="progressbar", and <button>s instead
  of <a href="#"> in the header.
- node:test suite (23 tests) covering the folder adapter, the Immich
  adapter (mocked fetch, no live server needed), and the HTTP API
  end-to-end; wired up as `npm test`.
- Also includes the keyboard shortcuts (up/down undo/skip), swipe stamps,
  and shortcuts modal from the previous session that hadn't been
  committed yet.

public/settings.html

@@ -15,9 +15,12 @@
     </div>
   </header>
   <div class="source-label">Choose what to swipe through, and how.</div>
-  <div id="adapterList"></div>
+  <fieldset class="adapter-fieldset">
+    <legend class="sr-only">Adapter</legend>
+    <div id="adapterList"></div>
+  </fieldset>
   <form id="settingsForm"></form>
-  <div class="settings-error" id="settingsError"></div>
+  <div class="settings-error" id="settingsError" role="alert" aria-live="assertive"></div>
 </div>
 <script src="settings.js"></script>
 </body>