Add a network self-check to Diagnostics

[easonxiang]

Fixes #189

What changed

• About → Diagnostics → Run network check (no new screen). Each item shows Pass, Problem or Can't verify, what was found and, where useful, a next step:
  • Receiver: running / off / stopped with an error (the error text)
  • Network: Wi-Fi (with SSID when Android reveals it) or Ethernet plus IPv4 address; other network types, no address yet, or no answer within 2 s are Can't verify
  • _airplay._tcp and _raop._tcp on their own: registered (port) / waiting for Android / failed (NsdManager code and meaning) / not advertised
  • AirPlay listener: the port the native receiver reported; a Problem when only the discovery probe is up; Can't verify while the receiver is off
  • Last discovery failure: service type, time and error code; still listed but Pass once both services have registered again
  • Phone and TV on the same network: always Can't verify, with guidance (same non-guest Wi-Fi, AP/client isolation off, no VPN on the phone)
• AirPlayDiscoveryAdvertiser keeps each registration's state and the latest failure (DiscoveryStatus), including after it stops itself on a failure, and records failures as discovery events. AirPlayManager.snapshot() exposes it with the receiver state and listener port.
• The check only reads state. It never stops or restarts AirPlay or DLNA. Each run is recorded as a check event.
• NetworkCheck.classify is pure and unit-tested. The UI maps its reasons to strings (English and Simplified Chinese).
• The Diagnostics header buttons now wrap (FlowRow) under the title, because three buttons in one row pushed Upload/Clear off a phone-width screen.

No restart action was added, as the issue asks.

Tested

• ./gradlew assembleDebug :app:testDebugUnitTest :app:lintVitalRelease passes. NetworkCheckTest has 9 tests covering classification and state mapping, and checks NsdManager codes against the SDK constants.
• Emulator (emulator-5554, Android 16, phone size), debug build via tools/sim:
  • Ran the check with a tap and with the D-pad (focus on the button, OK, then down through each result row).
  • Results: receiver, Wi-Fi 10.0.2.16, both services on port 7000, listener 7000 and no failures all Pass. The same-network item shows Can't verify.
  • The log shows no receiver stop or start when the check ran.
• Not exercised on a device: the failure and discovery-only paths (unit tests cover them) and the Chinese strings on screen.

Device checks left for review

• On a TV (Google TV, and the Android 6 Xiaomi for the API 23 path), with the remote: open Diagnostics, run the check, and scroll through the results.
• Run the check while an iPhone is mirroring or playing music, and confirm the cast continues.
• Look at the results in Chinese.
• On a car head unit or tablet: Ethernet / Wi-Fi detection, and whether the SSID appears when the location permission is granted.

🤖 Generated with Claude Code