[codex] Stabilize offline-first Flutter test suite

[berlogabob]

Summary

Stabilizes the offline-first Flutter test suite and keeps the canonical follow-up plan under plans/remaining_test_stabilization_plan.md.

Changes include:

• fixing offline edit save repo routing so owner/repo are preserved separately
• adding test-safe initialization hooks for ErrorLoggingService
• hardening ErrorBoundary retry, reporting, inline detail, and overflow behavior
• updating stale widget/service tests to match the current offline-first UI and loader behavior
• removing the duplicated top-level PLAN20260527.md in favor of the canonical plans/ file

Stack

This PR is stacked on #50 (codex/offline-first-base). Review and merge #50 first, then rebase/update this PR onto main.

Toolchain

Validated after rebasing on the base branch toolchain update:

• Flutter 3.44.0 stable
• Dart 3.12.0

Verified Gates

• /tmp/flutter-3.44.0/flutter/bin/flutter analyze
• /tmp/flutter-3.44.0/flutter/bin/flutter build apk --debug
• /tmp/flutter-3.44.0/flutter/bin/flutter test --reporter compact --concurrency=1 (907 tests passed)