feat: resumable transfer UX — surface partial downloads to UI on reconnect

Daemon scans the download directory for .tmp.meta sidecars on network join
and emits resumable_transfers IPC event. Web UI shows them in the Transfers
panel with a dimmed progress bar and "will resume on reconnect" note.

- proto: ResumableFile type, EvtResumableTransfers, resumable_files IpcMessage field
- mesh: ScanResumable() scans download dir and emits the event
- netmgr: call ScanResumable() after join (both Join and JoinByHash paths)
- web: resumableFiles store state, resumable_transfers handler, Transfers UI section

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/types.ts

@@ -85,6 +85,7 @@ export type IpcMsgType =
   | 'history_loaded'
   | 'room_created'
   | 'create_room'
+  | 'resumable_transfers'
 
 export interface IpcMessage {
   type: IpcMsgType
@@ -129,4 +130,6 @@ export interface IpcMessage {
   remote_address?: string
   // history_loaded
   messages?: ChatMessage[]
+  // resumable_transfers
+  resumable_files?: Array<{ name: string; sha256: string; from: string; size: number; offset: number }>
 }