Add TransferTribe app and fix critical transfer security flaws

The application source was untracked, so this commit brings it under
version control together with fixes for the issues found while auditing
it. Notable fixes:

Authorization
- Require a session and scope to senderEmail on /api/transfers/[id]
  (GET, DELETE) and .../resend. These were unauthenticated over an
  autoincrement id, so the ids could be walked to soft-delete any
  transfer, read sender/recipient metadata, or make the app mail
  arbitrary recipients.
- Require a session on the legacy /api/send and /api/chunk-upload
  endpoints, and take the sender from the session rather than a request
  field so transfers cannot be posted as another user.

Encryption
- Replace the chunk encryption scheme. Every chunk was encrypted under
  one shared session IV with its auth tag discarded, which reuses the
  AES-GCM keystream (XORing two ciphertexts recovers plaintext without
  the key) and left the stored file undecryptable, surfacing to users as
  a wrong-password error. Chunks are now self-contained frames carrying
  their own random IV and auth tag, behind a magic+salt header.
- Files written by the previous format now report UNSUPPORTED_FORMAT
  instead of a misleading password error.

Download
- Verify the password against the stored bcrypt hash before serving a
  file, and enforce expiresAt and DELETED/EXPIRED status.
- Move the password from the query string into a POST body so it stays
  out of access logs and Referer headers.
- Record a download only after successful authentication.
- Decrypt frame by frame through a stream instead of buffering the whole
  file, and encode the Content-Disposition filename per RFC 5987.

Data exposure
- /api/download ran before the password prompt and returned the full
  transfer row, including absolute server file paths. It now returns
  only what the pre-password screen renders; filenames, message and
  recipient are withheld until /api/verify succeeds.

Correctness
- Fix BigInt handling that made /api/transfers and /api/transfers/[id]
  fail unconditionally (JSON.stringify cannot serialize BigInt, and
  seeding a BigInt reduce with 0 throws).
- Fail loudly on a missing chunk during reassembly rather than silently
  writing a corrupt file.
- Meter plan usage in plaintext bytes rather than on-disk encrypted size.

Ignore /uploads: it holds runtime transfer payloads, not source.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

components.json

@@ -0,0 +1,21 @@
+{
+  "$schema": "https://ui.shadcn.com/schema.json",
+  "style": "default",
+  "rsc": true,
+  "tsx": true,
+  "tailwind": {
+    "config": "",
+    "css": "src/app/globals.css",
+    "baseColor": "neutral",
+    "cssVariables": true,
+    "prefix": ""
+  },
+  "aliases": {
+    "components": "@/components",
+    "utils": "@/lib/utils",
+    "ui": "@/components/ui",
+    "lib": "@/lib",
+    "hooks": "@/hooks"
+  },
+  "iconLibrary": "lucide"
+}