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.

CONTRIBUTING.md

@@ -45,11 +45,29 @@ work for any adapter that implements the contract below.
        // Reverse applyAction() using the record it returned.
      }
 
-     // Optional, only if items have a file-like preview:
-     async resolvePreviewPath(itemId) {
+     // Optional: write a full-size preview straight to the Express response.
+     // Return true if you handled it, false to let the server 404.
+     async streamPreview(itemId, res) {
+       return false;
+     }
+
+     // Optional: same, but for a smaller/faster thumbnail (used for the
+     // card image and as a video poster frame). Falls back to streamPreview
+     // in the UI if omitted or it returns false.
+     async streamThumbnail(itemId, res) {
+       return false;
+     }
+
+     // Optional: describe a reversible "trash" so the UI offers a
+     // confirm-guarded "Empty trash" action. Return null if you don't have one.
+     async describeTrash() {
        return null;
      }
 
+     async emptyTrash() {
+       throw new Error('not supported');
+     }
+
      describeSource() {
        return ''; // short string shown in the UI header
      }
@@ -58,6 +76,11 @@ work for any adapter that implements the contract below.
    module.exports = { YourAdapter };
    ```
 
+   See `adapters/folder.js` for a local-filesystem implementation (`res.sendFile`
+   + a Quick Look thumbnail cache) and `adapters/immich.js` for a remote-API
+   implementation (proxying a fetch response straight into `res`) of
+   `streamPreview`/`streamThumbnail`.
+
 2. Register it in `adapters/registry.js`:
 
    ```js
@@ -97,6 +120,27 @@ npm start
 There's no build step — edit files under `public/` or `adapters/` and
 refresh the browser.
 
+## Testing
+
+```bash
+npm test
+```
+
+Runs Node's built-in test runner (`node --test`) against `test/`. No extra
+dev dependencies, no separate test server to start.
+
+- `test/folder-adapter.test.js` exercises the filesystem adapter against real
+  temp directories (list/reject/undo/emptyTrash/path-traversal guard).
+- `test/immich-adapter.test.js` exercises the API-based adapter with
+  `global.fetch` mocked out — no live Immich server needed. Follow this
+  pattern for other network-backed adapters.
+- `test/api.test.js` boots the real Express app (`server.start(0)`) against an
+  isolated config/session file (via the `SWIPEANYTHING_CONFIG_PATH` /
+  `SWIPEANYTHING_SESSION_PATH` env vars) and drives it over real HTTP.
+
+Please add or extend tests for new adapters and API changes — a fetch-mocked
+adapter test is usually enough; you don't need a live backend to contribute one.
+
 ## Pull requests
 
 Keep adapters self-contained in their own file. Include a short section in