feat(desktop): in-app Diagnostics — bounded log capture, verbose toggle, scrubbed export (#180)

[badbread]

Desktop half of #180. Context that motivated doing this now: today's LPR black-screen mystery died unexplained precisely because the client keeps no logs — recon confirmed the desktop app had zero logging: no seam, no FlutterError.onError, empty catch (_) {} everywhere. This turns "something broke, no idea why" into an attachable file.

What

• DiagnosticsService (new): app-scoped bounded ring buffer (4k lines, capped message length, memory-only until export — nothing written to disk unasked). Warnings/errors/lifecycle always captured; a persisted verbose toggle (off by default) adds HTTP traces + mpv player logs. Bounded by construction, so it's safe to leave on while reproducing.
• Scrubbing at capture time (the issue's hard constraint): JWT shapes, Bearer values, token/password/secret/api_key query params and JSON fields are redacted before storage — nothing token-shaped ever even sits in the buffer — plus a second pass over the assembled export. Server identity is recorded as host:port only. Every redaction shape is pinned by unit tests (test/diagnostics_scrub_test.dart, 5/5 green on the build box).
• Global error funnels in main(): FlutterError.onError (chained to the default) + PlatformDispatcher.onError.
• HTTP trace at the single choke point (TimeoutClient.send): method + bare path + status + duration only — never query strings (?token=), headers, or bodies. Failures always; routine traffic verbose-only.
• Player hooks (live tile, maximized pane, playback panes): stream.error always; mpv's own log stream verbose-only. This is exactly the evidence that was missing for the LPR incident.
• Settings → Diagnostics pane: verbose toggle, Export logs… (native save dialog via the fullscreen picker guard), Copy to clipboard, live line count. Export header carries app version (package_info), OS, server host, verbose state.

Scope

Desktop only. The Android share-sheet export and the server-side admin diagnostics bundle from #180 are follow-ups — the issue stays open to track them.

Verify

• flutter analyze clean; flutter test test/diagnostics_scrub_test.dart 5/5.
• On the wall: Settings → Diagnostics → flip verbose, browse a bit, Export — the file opens with the header + scrubbed lines; grep it for eyJ/Bearer /token= and find only [REDACTED].

Part of #180 (leave open for Android + server bundle).