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/Cargo.toml

@@ -52,6 +52,7 @@ anyhow        = { workspace = true }
 thiserror     = { workspace = true }
 bincode       = { workspace = true }
 serde         = { workspace = true }
+serde_json    = { workspace = true }
 
 # CLI
 clap          = { workspace = true }