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>

cmd/daemon/main.go

@@ -21,6 +21,7 @@ func main() {
 	wsPort         := flag.Int("ws-port", 0, "port for WebSocket IPC (web UI); 0 = disabled")
 	anchorURL      := flag.String("anchor", "", "anchor WebSocket URL, e.g. ws://your-vps:17339/ws")
 	shareDir       := flag.String("share-dir", "", "directory to share with peers on the network")
+	downloadDir    := flag.String("download-dir", "", "base directory for received files; defaults to data-dir, split by network")
 	joinInvite     := flag.String("join", "", "waste: invite string — sets anchor URL and auto-joins the network on startup")
 	turnURL        := flag.String("turn-url", "", "TURN server URL, e.g. turn:your-vps:3478")
 	turnSecret     := flag.String("turn-secret", "", "shared secret for coturn use-auth-secret HMAC credential")
@@ -76,6 +77,7 @@ func main() {
 		StoreDir:       dir,
 		AnchorURL:      *anchorURL,
 		ShareDir:       expandHome(*shareDir),
+		DownloadDir:    expandHome(*downloadDir),
 		TurnURL:        *turnURL,
 		TurnSecret:     *turnSecret,
 	})