feat: direct-to-R2 Files uploads (client-sealed SPENC2) + chunked video dropzone + multi-file queue

For scale: payload bytes go browser → R2 → browser; the api handles keys,
authz and metadata only, so a cheap box can serve thousands of users.

- SPENC2 frame (browser-sealed, P-256 ECDH → HKDF wrap of a per-file CEK,
  10 MiG AES-GCM segments): storage.ts gains the reader (magic-branched
  getObject), presignPut/headObject, openDirectHeader; sealForDirect is the
  reference sealer for cross-implementation tests
- Files API: GET /files/direct/config, POST /files/direct/init (presigned
  PUT), POST /files/direct/complete (HEAD-gated commit), POST
  /files/:id/direct-download (presigned GET + CEK ticket)
- web: lib/direct-storage (WebCrypto seal/open), lib/files-direct (flows),
  dropzone rewritten with a multi-file sequential queue (per-file status,
  direct-first with server fallback), file download decrypts in-browser for
  direct objects (falls back to streaming)
- video dropzone wired to the chunked ingest API (>10 MiB → init/chunk/
  complete), fixing the Cloudflare ~100 MB body cap for large videos
- DIRECT_EC_PRIVATE_KEY (P-256, PKCS8 b64) wired through compose; cross-impl
  roundtrip tests prove web-seal ↔ server-open byte compatibility

apps/web/src/app/(main)/files/page.tsx

@@ -14,6 +14,7 @@ import { UploadDropzone } from "../../../components/files/upload-dropzone";
 import { ShareDialog } from "../../../components/files/share-dialog";
 import { RetentionDialog } from "../../../components/files/retention-dialog";
 import { downloadGroupFile } from "../../../components/files/files-client";
+import { downloadFileDirect, isDirectObject, saveBytes } from "../../../lib/files-direct";
 import { useFilesLang } from "../../../components/files/i18n";
 import { MIME_CLASS_LABEL } from "../../../components/files/labels";
 import type {
@@ -171,8 +172,23 @@ export default function FilesPage(): React.ReactElement {
   async function handleDownload(item: GroupFile): Promise<void> {
     setActionError(null);
     try {
+      if (isDirectObject(item)) {
+        const bytes = await downloadFileDirect(item.id);
+        saveBytes(bytes, item.name);
+        return;
+      }
       await downloadGroupFile(item.id, item.name);
     } catch (err) {
+      // Direct path is best-effort: fall back to streaming via the api once.
+      if (isDirectObject(item)) {
+        try {
+          await downloadGroupFile(item.id, item.name);
+          return;
+        } catch (fallbackErr) {
+          setActionError(errorMessage(fallbackErr));
+          return;
+        }
+      }
       setActionError(errorMessage(err));
     }
   }