Resumable saves: ranged downloads that survive dropped connections, reloads and app kills

Server
- /api/download/:id answers Range with a strong ETag ("<id>.<gen>") and honours
  If-Range; a stale partial gets the whole current file (streamed, so Bun does
  not re-apply the Range itself). Uploads get the same treatment.
- GET /api/download/:id/prepare never blocks: ready {gen,size,sha256,etag,ext},
  working (server still fetching), legacy (HEVC / too long / cache offline),
  failed. Recently refused prepares are remembered for 10 minutes.

Browser
- The OPFS worker saves in 8 MiB ranges, writes at the byte offset, flushes
  each chunk, retries each chunk 6 times with backoff (30 s idle timeout) and
  keeps the .part plus a .part.json sidecar naming the server copy it belongs
  to. A changed copy restarts cleanly; the finished file is hashed once and
  checked against the server's SHA-256.
- SaveQueue remembers unfinished saves and resumes them on start, online,
  return to the foreground and every 2 minutes while visible; one at a time.
- Downloads shows live MB progress, 'Preparing on server', 'Verifying', and
  paused saves with Resume and Cancel (confirmed).
- listVideos ignores the sidecars; new listPartials/discardPartial helpers.

Verified in Chromium through a connection-dropping proxy: paused at 8 MiB,
auto-resumed after a reload from byte 8388608, final SHA-256 matched.

frontend/opfs.js

@@ -145,7 +145,9 @@
     // workers. Resolves { ok:true } on success, { ok:false, error } on a real
     // failure, or { ok:false, fallback:true } when the worker path is
     // unavailable and the caller should use writeFromResponse instead.
-    downloadVideo(videoId, url) {
+    // opts.resumable = { etag, size, sha256, ext } switches the worker to the
+    // ranged, resumable save; opts.onProgress(received, total, info) reports.
+    downloadVideo(videoId, url, opts = {}) {
       return new Promise((resolve) => {
         let worker;
         try {
@@ -163,10 +165,14 @@
           if (m.type === 'done')             finish({ ok: true, sha256: m.sha256 || null, expectedSha: m.expectedSha || null, size: m.size || 0 });
           else if (m.type === 'unsupported') finish({ ok: false, fallback: true });
           else if (m.type === 'error')       finish({ ok: false, error: m.error });
-          // 'progress' messages are informational; ignored here
+          else if (m.type === 'paused')      finish({ ok: false, paused: true, received: m.received || 0, error: m.error });
+          else if (m.type === 'changed')     finish({ ok: false, changed: true });
+          else if (m.type === 'progress' && opts.onProgress) {
+            try { opts.onProgress(m.received || 0, m.total || 0, m); } catch { /* UI only */ }
+          }
         };
         worker.onerror = () => finish({ ok: false, fallback: true });
-        worker.postMessage({ videoId, url });
+        worker.postMessage({ videoId, url, resumable: opts.resumable || null });
       });
     },
 
@@ -249,8 +255,8 @@
       try {
         for await (const [name, handle] of dir.entries()) {
           if (handle.kind !== 'file') continue;
-          // Skip .part temporary files
-          if (name.endsWith('.part')) continue;
+          // Skip .part temporary files and resumable-save sidecars
+          if (name.endsWith('.part') || name.endsWith('.part.json')) continue;
           const dot = name.lastIndexOf('.');
           const id = dot > -1 ? name.slice(0, dot) : name;
           const file = await handle.getFile();
@@ -260,6 +266,37 @@
       return items;
     },
 
+    // Paused resumable saves: [{ id, received, size }] from <id>.<ext>.part
+    // and its .part.json sidecar (written by opfs-worker.js).
+    async listPartials() {
+      const out = [];
+      try {
+        const dir = await getRoot();
+        for await (const [name, handle] of dir.entries()) {
+          if (handle.kind !== 'file' || !name.endsWith('.part.json')) continue;
+          const partName = name.slice(0, -5);
+          let meta = null, received = 0;
+          try { meta = JSON.parse(await (await handle.getFile()).text()); } catch { /* unreadable */ }
+          try { received = (await (await dir.getFileHandle(partName)).getFile()).size; } catch { /* no data yet */ }
+          const id = partName.replace(/\.[^.]+\.part$/, '');
+          out.push({ id, received, size: (meta && meta.size) || 0 });
+        }
+      } catch { /* OPFS not available */ }
+      return out;
+    },
+
+    // Drop a paused save's partial data (Cancel in Downloads).
+    async discardPartial(videoId) {
+      try {
+        const dir = await getRoot();
+        const names = [];
+        for await (const [name] of dir.entries()) {
+          if ((name.endsWith('.part') || name.endsWith('.part.json')) && name.startsWith(videoId + '.')) names.push(name);
+        }
+        for (const name of names) await dir.removeEntry(name).catch(() => {});
+      } catch { /* OPFS not available */ }
+    },
+
     // Total bytes stored
     async totalSize() {
       const items = await this.listVideos();