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.

lib/thumbnails.js

@@ -0,0 +1,84 @@
+'use strict';
+
+// Thumbnail generation for the local preview endpoint. Uses macOS's built-in
+// Quick Look generator (`qlmanage`) so we get resized previews, video poster
+// frames, and HEIC/HEIF support for free -- no extra npm dependencies, no
+// native binaries to compile. On non-macOS platforms this silently no-ops
+// and callers fall back to serving the original file.
+
+const os = require('os');
+const fs = require('fs');
+const fsp = fs.promises;
+const path = require('path');
+const crypto = require('crypto');
+const { execFile } = require('child_process');
+
+const CACHE_DIR = path.join(__dirname, '..', '.cache', 'thumbnails');
+// qlmanage/quicklookd can hang indefinitely in some environments (headless
+// sessions, SSH, sandboxes without WindowServer access). Keep this short and
+// always hard-kill on timeout -- worse case is we fall back to the original
+// file a little slower, never a hung request.
+const GENERATE_TIMEOUT_MS = 3000;
+const IS_MACOS = process.platform === 'darwin';
+
+let cacheDirReady = null;
+function ensureCacheDir() {
+  if (!cacheDirReady) {
+    cacheDirReady = fsp.mkdir(CACHE_DIR, { recursive: true });
+  }
+  return cacheDirReady;
+}
+
+function cacheKeyFor(absPath, mtimeMs) {
+  return crypto.createHash('sha1').update(`${absPath}:${mtimeMs}`).digest('hex');
+}
+
+/**
+ * Returns an absolute path to a cached thumbnail PNG for `absPath`, or null
+ * if thumbnails aren't supported on this platform / generation failed.
+ * Safe to call for any file type; unsupported types just fail fast.
+ */
+async function getThumbnail(absPath, { size = 480 } = {}) {
+  if (!IS_MACOS) return null;
+
+  const stat = await fsp.stat(absPath).catch(() => null);
+  if (!stat) return null;
+
+  await ensureCacheDir();
+  const key = cacheKeyFor(absPath, stat.mtimeMs);
+  const cachedPath = path.join(CACHE_DIR, `${key}.png`);
+
+  if (fs.existsSync(cachedPath)) return cachedPath;
+
+  const tmpDir = await fsp.mkdtemp(path.join(os.tmpdir(), 'swipeanything-thumb-'));
+  try {
+    await new Promise((resolve, reject) => {
+      const child = execFile(
+        'qlmanage',
+        ['-t', '-s', String(size), '-o', tmpDir, absPath],
+        { timeout: GENERATE_TIMEOUT_MS, killSignal: 'SIGKILL' },
+        (err) => (err ? reject(err) : resolve())
+      );
+      // Belt-and-suspenders: qlmanage/quicklookd have been observed to hang
+      // past Node's own `timeout` option in some sandboxed environments.
+      const hardKill = setTimeout(() => {
+        child.kill('SIGKILL');
+        reject(new Error('qlmanage timed out'));
+      }, GENERATE_TIMEOUT_MS + 500);
+      child.once('exit', () => clearTimeout(hardKill));
+    });
+
+    const base = path.basename(absPath);
+    const generated = path.join(tmpDir, `${base}.png`);
+    if (!fs.existsSync(generated)) return null;
+
+    await fsp.rename(generated, cachedPath);
+    return cachedPath;
+  } catch {
+    return null;
+  } finally {
+    fsp.rm(tmpDir, { recursive: true, force: true }).catch(() => {});
+  }
+}
+
+module.exports = { getThumbnail, CACHE_DIR };