feat: Sprint 5 — encrypted file transfer with chunked upload/download

- Add uploadBlob (@21) and downloadBlob (@22) RPCs to Cap'n Proto
  schema with SHA-256 content addressing and chunked transfer
- Server blob handler: 256KB chunks, SHA-256 verification on finalize,
  .meta JSON sidecar, 50MB size limit, content-addressable storage
- Add FileRef (0x08) AppMessage variant with blob_id, filename,
  file_size, mime_type
- /send-file command: read file, compute hash, upload in chunks with
  progress display, send FileRef via MLS, MIME auto-detection
- /download command: fetch blob in chunks with progress, verify hash,
  save to disk with collision avoidance
- 2 new E2E tests: upload/download round-trip with partial reads,
  hash mismatch rejection (14 E2E tests total)
- New error codes: E024-E027 for blob operations

crates/quicproquo-server/src/error_codes.rs

@@ -26,6 +26,10 @@ pub const E020_BAD_PARAMS: &str = "E020";
 pub const E021_CIPHERSUITE_NOT_ALLOWED: &str = "E021";
 pub const E022_CHANNEL_ACCESS_DENIED: &str = "E022";
 pub const E023_CHANNEL_NOT_FOUND: &str = "E023";
+pub const E024_BLOB_TOO_LARGE: &str = "E024";
+pub const E025_BLOB_HASH_LENGTH: &str = "E025";
+pub const E026_BLOB_HASH_MISMATCH: &str = "E026";
+pub const E027_BLOB_NOT_FOUND: &str = "E027";
 
 /// Build a `capnp::Error::failed()` with the structured code prefix.
 pub fn coded_error(code: &str, msg: impl std::fmt::Display) -> capnp::Error {