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>

web/public/manifest.json

@@ -0,0 +1,23 @@
+{
+  "name": "waste",
+  "short_name": "waste",
+  "description": "Decentralized friend-to-friend encrypted mesh networking",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#0d0d0d",
+  "theme_color": "#863bff",
+  "icons": [
+    {
+      "src": "/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any maskable"
+    }
+  ]
+}