feat: download dirs, file transfer resume, Wails desktop app, PWA, CI

Daemon:
- Per-network download directories (-download-dir flag, set_download_dir IPC)
- File transfer resume after disconnection: .tmp.meta sidecars survive
  interruption; resume_offset in file-accept lets sender seek and continue
- set_download_dir IPC command; download_dir reported in state_snapshot

Protocol:
- PeerMessage.ResumeOffset (EXT-006) for file transfer resume
- IpcMessage.ShareNetworks json tag changed from "networks" to "network_ids"
  to fix duplicate json tag collision with Networks []NetworkInfo

Desktop app (cmd/app):
- Wails v2 shell embedding daemon logic directly (no subprocess)
- System tray on Linux/Windows via getlantern/systray; macOS hides to Dock
- OS notifications for message_received and file_complete via Wails events
- notray build tag for headless/CI builds without GTK tray headers
- build-app.sh: builds web frontend, copies dist, runs wails build

Web / PWA:
- manifest.json + Apple touch icon meta tags for mobile "Add to Home Screen"
- PNG icons (192px, 512px, 180px) generated from SVG
- Wails EventsOn("notify") hook in App.tsx for native OS notifications

CI:
- .gitea/workflows/build.yml: server binaries cross-compiled for 5 platforms,
  desktop app for Linux amd64, release artifacts published on v* tags

Docs:
- README: download dir, file transfer resume, desktop app, PWA, CI sections
- EXTENSIONS.md: EXT-004 daemon mode marked shipped; EXT-006 resume added
- FUTURE.md: roadmap updated

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

internal/proto/proto.go

@@ -73,10 +73,11 @@ type PeerMessage struct {
 	FileListResp *FileListResp `json:"file_list_resp,omitempty"`
 
 	// file transfer (§9) — fields are flat on the wire
-	Xid    string `json:"xid,omitempty"`
-	Name   string `json:"name,omitempty"`
-	Size   int64  `json:"size,omitempty"`
-	SHA256 string `json:"sha256,omitempty"`
+	Xid          string `json:"xid,omitempty"`
+	Name         string `json:"name,omitempty"`
+	Size         int64  `json:"size,omitempty"`
+	SHA256       string `json:"sha256,omitempty"`
+	ResumeOffset int64  `json:"resume_offset,omitempty"` // waste-go ext: non-zero in file-accept when resuming
 
 	// file-done / file-cancel / file-accept just need xid (already above)
 	Reason string `json:"reason,omitempty"` // file-cancel
@@ -236,7 +237,8 @@ const (
 	CmdAddShare       IpcMsgType = "add_share"        // add a share root; fields: path, networks
 	CmdRemoveShare    IpcMsgType = "remove_share"      // remove a share root; field: path
 	CmdListShares     IpcMsgType = "list_shares"       // returns shares_list event
-	CmdCreateRoom     IpcMsgType = "create_room"       // field: room (name)
+	CmdCreateRoom       IpcMsgType = "create_room"        // field: room (name)
+	CmdSetDownloadDir   IpcMsgType = "set_download_dir"   // set per-network download directory at runtime; fields: network_id, path
 
 	// Events (daemon → UI)
 	EvtMessageReceived  IpcMsgType = "message_received"
@@ -312,7 +314,7 @@ type IpcMessage struct {
 	InviteGenerated string      `json:"invite,omitempty"`
 	Files          []FileEntry  `json:"files,omitempty"`
 	Shares         []ShareEntry `json:"shares,omitempty"`
-	ShareNetworks  []string     `json:"networks,omitempty"` // for add_share command
+	ShareNetworks  []string     `json:"network_ids,omitempty"` // for add_share command: scope to specific network IDs, or ["*"] for global
 	// export_identity / import_identity
 	Passphrase     string       `json:"passphrase,omitempty"` // import only; never echoed back
 	Backup         string       `json:"backup,omitempty"`     // JSON backup blob