feat(ui): add warning banner when repository has 0 commits in analyzed range (#208)

[Myparadox-creator]

Summary of Changes

Closes

#208

Problem

If a repository exists but has 0 commits in the analyzed range (or when a date filter excludes all commits), the dashboard previously displayed empty or broken states without guidance advising the user on how to resolve the issue.

Solution

1. EmptyCommitsWarningBanner Component:

   • Created frontend/src/components/EmptyCommitsWarningBanner.tsx with a glassmorphic amber banner, icons, clear titles, and context-aware messaging.
   • Dynamically detects whether 0 commits is due to an active time filter (7d, 30d, 1y, custom) vs 0 analyzed commits in the repository overall.
   • Provides quick-action buttons:
     • Reset to All Time: resets the time range filter when a filter is active.
     • Update Analysis: triggers a repository rescan.
     • Adjust Settings: navigates to the landing page to re-ingest with higher max commits or a different branch.
   • Built with accessibility in mind (role="alert" and data-testid="empty-commits-warning-banner").

2. Dashboard Integration:

   • Integrated <EmptyCommitsWarningBanner /> in frontend/src/pages/DashboardPage.tsx when !timelineState.isLoading && commits.length === 0.
   • Wired callbacks for time filter resetting, rescan, and re-ingestion.

3. Testing & Validation:

   • Added unit test suite in frontend/src/components/EmptyCommitsWarningBanner.test.tsx.
   • Expanded frontend/src/pages/DashboardPage.test.tsx to verify warning banner rendering and reset interaction.
   • 100% test pass rate (121 frontend tests across 35 files; 287 backend tests).

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