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/ipc/ipc.go

@@ -344,6 +344,22 @@ func handleClient(conn net.Conn, mgr *netmgr.Manager) {
 				continue
 			}
 
+		case proto.CmdSetDownloadDir:
+			n := mgr.Resolve(cmd.NetworkID)
+			if n == nil {
+				send(errMsg("set_download_dir: not joined to any network"))
+				continue
+			}
+			if cmd.Path == "" {
+				send(errMsg("set_download_dir: path is required"))
+				continue
+			}
+			if !mgr.SetDownloadDir(n.ID, cmd.Path) {
+				send(errMsg("set_download_dir: network not found"))
+				continue
+			}
+			send(stateSnapshot(mgr))
+
 		case proto.CmdSendFile:
 			n := mgr.Resolve(cmd.NetworkID)
 			if n == nil {