feat(camera): in-app diagnostic for "Connection lost" (#1395 follow-up)

Step 2 of the camera architecture overhaul agreed after #1395. When
  the camera viewer hits its error state OR before a print at any
  time, a Diagnose button runs a staged check against the printer and
  renders the result inline: which stage failed, how long it took,
  and a translated remediation hint. Cuts off the "user opens a
  'camera broken' ticket → ask for support bundle → triage" loop at
  the user's screen.

  Backend

  - New `backend/app/services/camera_diagnose.py` orchestrator with
    CameraDiagnoseResult / CameraDiagnoseStage dataclasses.
  - New POST /printers/{id}/camera/diagnose route in camera.py.
  - Stages:
      tcp_reachable — TCP socket open to 322 (RTSP) / 6000 (chamber)
        with 3 s timeout. Distinguishes timeout, refused, and host-
        unreachable into distinct summary codes so the frontend can
        show a precise remediation (firewall vs LAN-only off vs
        wrong IP).
      first_frame — captures one JPEG end-to-end via the existing
        capture_camera_frame_bytes pipeline. Auth + RTSP handshake +
        first keyframe collapse into one stage; the user-facing
        answer is the same regardless of which sub-layer failed.
  - Live-stream shortcut: when a viewer is currently watching the
    camera with a buffered frame < 10 s old, the diagnostic skips
    the real test and returns live_stream_active_healthy. Opening a
    fresh socket would kick the live viewer off on single-camera-
    connection firmwares (the #1348 reconnect-storm trigger), so we
    trust the real-world evidence instead.
  - Response surfaces protocol, port, and profile name for support
    triage — lets us ask "what does your modal say?" instead of
    "send the support bundle".

  Frontend

  - New CameraDiagnoseModal renders one row per stage with green-
    check / red-X / grey-skipped icons, the per-stage duration in
    ms, a remediation banner styled by overall status, and a Run
    again button.
  - Two entry points:
      1. The viewer's error overlay grows a Diagnose button next to
         Retry. Retry stays the primary action; Diagnose is the
         escape hatch for users who can't see what's wrong.
      2. A stethoscope icon in the viewer's always-visible control
         bar, between Refresh and Fullscreen. Pre-flight testing
         ("did my firmware update break the camera?", "is the
         camera up before I send a print?") doesn't require waiting
         for the stream to fail first.
  - Also lifted the previously-hard-coded "Camera unavailable" /
    "Retry" strings into camera.unavailable / camera.retry so the
    error UI is fully translated alongside the new keys.

frontend/scripts/check-i18n-parity.mjs

@@ -164,6 +164,7 @@ const DE_COGNATES = [
   'Avery L7160 — A4 sheet (38.1 × 63.5 mm × 21)',
   'Avery 5160 — US Letter sheet (25.4 × 66.7 mm × 30)',
   'China', 'Proxy', 'Start',
+  'Diagnose',  // DE: same spelling/meaning as EN — camera diagnostic button label
 ];
 
 // French cognates — many UI labels overlap with English exactly.