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/mesh/transfer.go

@@ -91,6 +91,47 @@ func writePartialMeta(path string, t *inboundTransfer) {
 	os.WriteFile(path, data, 0o644) //nolint:errcheck
 }
 
+// ScanResumable scans the download directory for .tmp.meta sidecars left by
+// interrupted transfers and emits a resumable_transfers IPC event listing them.
+// Called once after a network is joined so the UI can show pending transfers.
+func (m *Mesh) ScanResumable() {
+	if m.DownloadDir == "" {
+		return
+	}
+	metas, _ := filepath.Glob(filepath.Join(m.DownloadDir, "*.tmp.meta"))
+	var files []proto.ResumableFile
+	for _, mp := range metas {
+		data, err := os.ReadFile(mp)
+		if err != nil {
+			continue
+		}
+		var meta partialMeta
+		if err := json.Unmarshal(data, &meta); err != nil {
+			continue
+		}
+		tp := strings.TrimSuffix(mp, ".meta")
+		info, err := os.Stat(tp)
+		if err != nil {
+			continue
+		}
+		files = append(files, proto.ResumableFile{
+			Name:   meta.Name,
+			SHA256: meta.SHA256,
+			From:   meta.From,
+			Size:   meta.Size,
+			Offset: info.Size(),
+		})
+	}
+	if len(files) == 0 {
+		return
+	}
+	m.emit(proto.IpcMessage{
+		Type:           proto.EvtResumableTransfers,
+		ResumableFiles: files,
+	})
+	log.Printf("transfer: %d resumable transfer(s) found in %s", len(files), m.DownloadDir)
+}
+
 // OfferFile reads filename from ShareDir, computes its SHA-256, and sends a
 // file-offer to peerID over the existing "yaw" DataChannel.
 func (m *Mesh) OfferFile(peerID proto.PeerID, filename string) error {