feat(studio,agent): mDNS auto-discovery for WiFi physical devices

[pbertsch]

Summary

Two-sided change so Studio can find physical iOS/Android devices on the local network without manual IP entry. Pairs with PR #85 (USB physical-device support) to give Studio its full physical-device story for v0.7.0.

Agent (flutter_probe_agent v0.7.0)

• New dep: bonsoir: ^5.1.10
• When started in WiFi mode (PROBE_WIFI=true), advertises itself as _flutterprobe._tcp with TXT records {version, port}. Localhost-bound deployments skip mDNS entirely so simulator-only apps pay zero overhead.
• Token deliberately NOT in TXT records — anyone on the LAN could read it. Users still paste from PROBE_TOKEN= log lines.
• Version bumped 0.6.0 → 0.7.0; CHANGELOG entry added.

Studio

• New dep: github.com/grandcat/zeroconf v1.0.0
• New file studio/wifi_discovery.go owns the browse loop. Discovered devices stream out via the wifi:device-found Wails event with dedup on host:port.
• New Wails methods: StartWiFiDiscovery, StopWiFiDiscovery, ConnectWiFi(host, port, token).
• Frontend: WiFi button (📡) in the toolbar opens a modal listing discovered devices. User clicks one, pastes the token, hits Connect. Modal close cancels the browse cleanly.
• Modal markup uses DOM construction (not innerHTML) for agent-supplied strings — agent name/host/version are user-controlled within the LAN trust boundary.

Compatibility

Studio v0.7.0 only discovers agents flutter_probe_agent v0.7.0+. Older agents continue to work over USB through the existing Connect path — no regression.

Release coordination

This PR ships changes to a published Dart package. After merge:

1. Tag and release triggers dart pub publish for flutter_probe_agent v0.7.0 (.github/workflows/publish-pub-dev.yml).
2. Studio v0.7.0 binary ships in the same release.
3. Users update both their app's pubspec.yaml (rebuild required) and their Studio binary.

Test plan

• go build ./... — root module clean
• go test ./... — full Go suite green
• cd studio && go build ./... && go vet ./... clean
• cd probe_agent && dart analyze lib/ — no issues
• cd probe_agent && flutter test — 13/13 pass
• cd probe_agent && dart pub publish --dry-run — passes (warning only about uncommitted state during local check)
• cd studio/frontend && tsc --noEmit clean
• Manual: physical iPhone over WiFi — start app with PROBE_WIFI=true, open Studio, click 📡, see device discovered, paste token, connect, see live screen
• Manual: physical Android over WiFi — same
• Manual: regression — start a sim, USB-connect a device, verify the existing Connect path still works