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>

internal/netmgr/manager.go

@@ -162,6 +162,8 @@ func (mgr *Manager) Join(name, shareDir string) (string, error) {
 		}()
 	}
 
+	go m.ScanResumable()
+
 	mgr.emit(proto.IpcMessage{
 		Type:        proto.EvtNetworkJoined,
 		NetworkID:   netID,
@@ -249,6 +251,8 @@ func (mgr *Manager) JoinByHash(netHash64, shareDir string) (string, error) {
 		}()
 	}
 
+	go m.ScanResumable()
+
 	mgr.emit(proto.IpcMessage{
 		Type:        proto.EvtNetworkJoined,
 		NetworkID:   netID,