Carry file size, thumbnail size, and deletion flag through decryptFile (closes #37)

Foundation unit for the cache/API design. Three fields arrived on the wire
but decryptFile dropped them:

- Live files now set `file.size` from `info.fileSize` and `thumbnail.size`
  from `info.thumbSize`, left `undefined` when the server omits `info`.
- A deleted file is returned as a new `EnteFileTombstone` (`id`,
  `collectionID`, `updationTime`, `isDeleted: true`) with no decryption,
  since the server no longer holds ciphertext for it.

decryptFile now returns `EnteFile | EnteFileTombstone`. Rather than making
EnteFile's structural fields optional — which would force `?.`/guards across
every consumer under strict tsc and break the build — a tombstone is a
distinct minimal type, and `EnteFile.isDeleted?: false` is the discriminant
(a live file is never deleted). This keeps all existing consumers untouched;
they still receive fully-populated `EnteFile` values.

client.ts is the only direct caller: it now routes every row through
decryptFile and drops results whose `isDeleted` is set. listFiles still
returns live files only, so its observable behaviour is unchanged.

Model: opus-4-8

TODO.md

@@ -18,6 +18,12 @@ Update the README API reference section to match the current implementation.
 
 # Completed Steps
 
+- 2026-09-22: Carried file size, thumbnail size, and the deletion flag through
+  `decryptFile` (issue 37, foundation for the cache/API design). Live files now
+  populate `file.size`/`thumbnail.size` from the server's `info` (left
+  `undefined` when the server omits it), and a deleted file returns an
+  `EnteFileTombstone` — `id`, `collectionID`, `updationTime`, `isDeleted` —
+  instead of decrypting ciphertext the server no longer holds.
 - 2026-08-10: Made `lint-once.test.ts` enforce what its header claims. It walked
   `make check` only, so it never read `Dockerfile` — the image CI builds through
   `script/cibuild` — and a second `prettier --check .` could be added there with