feat(logging): one log file with rotation, and Open logs / Copy diagnostics in Help #1760

Problem

The app has no persistent log. Everything goes to `debugPrint` (about 77 call sites with ad-hoc prefixes, plus `logIgnored` with 54). The only file logs are the extension sandbox ones (`security_audit.log`, `.log`). `FlutterError.onError` and `runZonedGuarded` in `main.dart` also only `debugPrint`. A user cannot send us what happened: the macOS connection-save bug (#1757) had to be diagnosed by running the app from a terminal. (Whether Finder-launched macOS builds make these lines reachable at all is unverified.)

Proposal

• A small `QueryaLog` in `lib/core/util` that writes timestamped lines (level, area, message, stack) to a file in the app data root (`AppDataRoot`, so portable mode stays portable), with size-based rotation (for example 5 files of 2 MB).
• `debugPrint` stays for the console; `logIgnored` and the global handlers in `main.dart` also write to the file.
• Never log passwords, connection strings or SQL text; redact the user part of URLs.
• Help menu: Open logs folder and Copy diagnostics (version, OS, last N log lines).
• Tests for rotation, redaction and a failing file sink (logging must never throw).

Part of the error-handling epic.