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/proto/proto.go

@@ -99,6 +99,15 @@ type PeerMessage struct {
 	HistoryDone bool           `json:"history_done,omitempty"`
 }
 
+// ResumableFile describes a partially-downloaded file found on daemon startup.
+type ResumableFile struct {
+	Name   string `json:"name"`
+	SHA256 string `json:"sha256"`
+	From   string `json:"from"`    // peer ID hex
+	Size   int64  `json:"size"`
+	Offset int64  `json:"offset"`  // bytes already received
+}
+
 // HistoryEntry is one message in a history_chunk response.
 type HistoryEntry struct {
 	Mid       string `json:"mid"`
@@ -291,7 +300,8 @@ const (
 	EvtIdentityImported IpcMsgType = "identity_imported"
 	EvtSharesList       IpcMsgType = "shares_list"
 	EvtRoomCreated      IpcMsgType = "room_created"    // field: room (name)
-	EvtHistoryLoaded    IpcMsgType = "history_loaded"  // fields: room, messages
+	EvtHistoryLoaded      IpcMsgType = "history_loaded"       // fields: room, messages
+	EvtResumableTransfers IpcMsgType = "resumable_transfers"  // field: resumable_files
 )
 
 // NetworkInfo summarises one joined network for state_snapshot and network_joined events.
@@ -346,7 +356,8 @@ type IpcMessage struct {
 	ErrorMessage   string       `json:"error_message,omitempty"`
 	InviteGenerated string      `json:"invite,omitempty"`
 	Files          []FileEntry    `json:"files,omitempty"`
-	Messages       []ChatMessage  `json:"messages,omitempty"` // history_loaded
+	Messages        []ChatMessage   `json:"messages,omitempty"`          // history_loaded
+	ResumableFiles  []ResumableFile `json:"resumable_files,omitempty"`   // resumable_transfers
 	Shares         []ShareEntry `json:"shares,omitempty"`
 	ShareNetworks  []string     `json:"network_ids,omitempty"` // for add_share command: scope to specific network IDs, or ["*"] for global
 	// export_identity / import_identity