feat(crash): privacy-respecting crash reporting via Gitea issue

Capture uncaught exceptions on-device and let the user submit them, by
hand, as a Gitea issue — no network access, no auto-upload (the app holds
no INTERNET permission). Closes prod-readiness item 10; the issue
templates also close item 7.

- CrashReporter: uncaught-exception handler installed first in
  CalendulaApp.onCreate so startup crashes are caught too. Persists an
  allowlist-only report (app/Android/device version, locale, time, stack
  trace — nothing else) to filesDir/crash, then chains to the previous
  handler so the process still dies normally. Crash-loop detection +
  markHealthy reset.
- buildCrashReport is pure/testable; CrashReportBuilderTest asserts the
  header is exactly the allowlisted lines (guards against PII creep).
- Surfacing: next-launch dialog showing the full report verbatim (the
  privacy backstop) with a dismissed-marker so it doesn't nag; a Settings
  "Report a problem" row; and a minimal standalone CrashReportActivity
  that MainActivity routes to on a startup crash-loop, kept clear of the
  Hilt graph / DataStore theme.
- submitCrashReport copies the report to the clipboard and opens the
  prefilled Gitea issues/new URL (long traces fall back to paste).
- .gitea/ISSUE_TEMPLATE: crash_report, bug_report, feature_request.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/ISSUE_TEMPLATE/crash_report.md

@@ -0,0 +1,26 @@
+---
+name: Crash report
+about: Report a crash. Calendula can capture this for you (Settings → Report a problem, or the prompt after a crash) — it copies the report to your clipboard and prefills this form.
+title: "Crash: "
+labels:
+  - bug
+  - crash
+---
+
+<!--
+Thanks for reporting a crash in Calendula!
+
+If the app prefilled this for you, the crash report is already below — just add
+what you were doing and submit. Otherwise, paste the report from your clipboard
+into the code block. The report contains only app/Android/device versions and the
+stack trace — no personal data or calendar content.
+-->
+
+### What happened
+
+
+### Crash report
+
+```
+(paste the crash report here)
+```