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/netmgr/manager.go

@@ -33,6 +33,7 @@ import (
 type Config struct {
 	MasterIdentity *crypto.Identity
 	StoreDir       string // base directory for per-network SQLite files
+	DownloadDir    string // base directory for received files; defaults to StoreDir if empty
 	AnchorURL      string // WebSocket anchor URL used for all networks
 	ShareDir       string // default share directory; overridden per network via Join or SetShareDir
 	TurnURL        string // optional TURN server URL, e.g. "turn:your-vps:3478"
@@ -119,7 +120,7 @@ func (mgr *Manager) Join(name, shareDir string) (string, error) {
 	} else if mgr.cfg.ShareDir != "" {
 		m.ShareDir = mgr.cfg.ShareDir
 	}
-	m.DownloadDir = filepath.Join(mgr.cfg.StoreDir, "downloads-"+netID_full)
+	m.DownloadDir = filepath.Join(mgr.downloadBase(), "downloads-"+netID_full)
 	capturedNetID := netID
 	m.ScanFiles = func() []proto.FileEntry { return mgr.ScanAllShares(capturedNetID) }
 	if ice := mgr.turnICEServers(); ice != nil {
@@ -207,7 +208,7 @@ func (mgr *Manager) JoinByHash(netHash64, shareDir string) (string, error) {
 	} else if mgr.cfg.ShareDir != "" {
 		m.ShareDir = mgr.cfg.ShareDir
 	}
-	m.DownloadDir = filepath.Join(mgr.cfg.StoreDir, "downloads-"+netID)
+	m.DownloadDir = filepath.Join(mgr.downloadBase(), "downloads-"+netID)
 	capturedNetID2 := netID
 	m.ScanFiles = func() []proto.FileEntry { return mgr.ScanAllShares(capturedNetID2) }
 	if ice := mgr.turnICEServers(); ice != nil {
@@ -293,6 +294,28 @@ func (mgr *Manager) SetShareDir(netID, path string) bool {
 	return true
 }
 
+// downloadBase returns the configured download base directory, falling back to StoreDir.
+func (mgr *Manager) downloadBase() string {
+	if mgr.cfg.DownloadDir != "" {
+		return mgr.cfg.DownloadDir
+	}
+	return mgr.cfg.StoreDir
+}
+
+// SetDownloadDir updates the download directory for an already-joined network.
+// Changes take effect for the next incoming file transfer on that network.
+func (mgr *Manager) SetDownloadDir(netID, path string) bool {
+	mgr.mu.RLock()
+	net, ok := mgr.networks[netID]
+	mgr.mu.RUnlock()
+	if !ok {
+		return false
+	}
+	net.Mesh.DownloadDir = path
+	log.Printf("netmgr: download dir for %q set to %q", net.Name, path)
+	return true
+}
+
 // LeaveAll leaves every joined network.
 func (mgr *Manager) LeaveAll() {
 	mgr.mu.RLock()