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>

build-app.sh

@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+# Build the waste desktop app (Wails + embedded React frontend).
+#
+# Prerequisites:
+#   go install github.com/wailsapp/wails/v2/cmd/wails@latest
+#   Node.js 20+
+#
+# Usage:
+#   ./build-app.sh           # production build → bin/waste (or bin/waste.exe)
+#   ./build-app.sh dev       # dev mode (hot-reload; requires Vite dev server)
+set -euo pipefail
+
+MODE="${1:-build}"
+
+echo "==> Building web frontend..."
+(cd web && npm install --silent && npm run build)
+
+echo "==> Copying frontend dist into app package..."
+rm -rf cmd/app/frontend/dist
+cp -r web/dist cmd/app/frontend/dist
+
+if [ "$MODE" = "dev" ]; then
+  echo "==> Starting Wails dev mode (start 'cd web && npm run dev' in another terminal)..."
+  (cd cmd/app && wails dev)
+else
+  echo "==> Running Wails build..."
+  mkdir -p bin
+  (cd cmd/app && wails build -o "$(pwd)/../../bin/waste")
+  echo "==> Done: bin/waste"
+fi