Add inline diff previews in work log

[tyulyukov]

Summary

• Implements inline diff preview component for displaying code changes directly in conversation
• Adds telemetry infrastructure with PostHog integration for anonymous usage analytics
• Records provider lifecycle events (session start/stop, turns, interrupts, etc.) for operational insights
• Adds user setting to control inline diff visibility
• Implements telemetry identifier generation from existing auth credentials (Codex/Claude) with fallback to installation-scoped anonymous ID

Testing

• New analytics service tests verify event batching and flushing across multiple PostHog batch requests
• Inline diff utility tests cover parsing hunks, calculating stats, and edge cases
• Integration test confirms startup heartbeat records thread and project counts
• ProviderService tests verify telemetry events fire on session start/recover and turn operations
• All existing test layers updated to provide AnalyticsService (test variant)
• Manual: Toggle showInlineDiffs setting in chat settings to verify inline diffs render/hide correctly