fix: resolve 106 remaining SonarQube LOW severity issues

[rygel]

Summary

• S1075 (35 issues): Extract hardcoded URIs/paths to const fields across 16 provider and service files. ProviderEndpoints.cs suppressed with #pragma warning disable S1075 since it is already a centralized constants file.
• S1192 (57 issues): Extract repeated string literals (3+ occurrences) to const string fields across 15 files, including 28 theme names in App.Themes.cs, JSON property names in CodexProvider.cs and OpenAIProvider.cs, SQL table names in DatabaseMigrationService.cs, and status strings across Monitor client files.
• S2325 (7 issues): Make non-instance methods static in ProviderUsageProcessingPipeline, App.Screenshots, SettingsWindow.Providers, ProviderRefreshConfigSelector, and Index.cshtml.
• S6664 (7 issues): Consolidate excessive debug logging calls in ZaiProvider, GitHubUpdateChecker, Program.cs (3 blocks), ProviderRefreshConfigLoadingService, and WebApplicationPipelineExtensions.
• S1481 (1 issue): False positive - the version variable in _Layout.cshtml is used in the Razor template footer.

Changes

• 35 files modified, ~1,300 lines touched
• No behavioral changes - only refactoring to satisfy SonarQube rules

Test plan

• dotnet build AIUsageTracker.sln - 0 errors
• dotnet test all 3 test projects - 1,491 passed, 0 failed
• Merge conflicts with develop resolved

[github-actions]

⚠️ Large PR Detected

This PR contains 1301 lines changed across 35 files.

Details:

• 📁 Files changed: 35
• ➕ Insertions: 694
• ➖ Deletions: 607
• 📊 Total: 1301 lines

Recommendation:
Consider splitting this PR into smaller, focused changes to make review easier and faster. PRs under 500 lines are ideal for thorough code review.

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