feat: git project drill-down from Projects view

[NovakPAai]

Summary

Adds the ability to drill into a specific git project from the Projects view and see only its sessions — instead of scrolling through a flat list of all sessions.

UX flow

1. Open Projects in the sidebar — see all repos with session count, message count, and estimated cost
2. Click Open › next to any project (e.g. tasktime-mvp)
3. Switch to Sessions view filtered to only that project's sessions
4. A breadcrumb bar appears: Project: tasktime-mvp × Clear filter
5. Click "Clear filter" → return to Projects view

Any sidebar navigation automatically clears the filter.

Technical notes

• Grouping by key, not name — renderProjects now groups by full git_root path instead of directory name, preventing collision when two repos share the same folder name
• Filter reset on drill-down — clears searchQuery, tagFilter, dateFrom, dateTo and their UI elements before filtering, so users don't land on an empty list due to a previously active date range
• XSS hardening — project key/name passed via data-proj-key / data-proj-name attributes and read via this.dataset in onclick, instead of inlining JSON.stringify() into the HTML attribute string
• groupingMode note — drill-down always uses getRepoInfo (git-root key); this is intentional and documented in a comment

Test plan

• Open Projects → click "Open ›" on any project → only that project's sessions appear
• Breadcrumb shows project name and "Clear filter" button
• Click "Clear filter" → returns to Projects view
• Click any sidebar item while filter is active → filter clears, full sessions shown
• Set a date filter, then drill into a project → date filter is reset, all project sessions visible
• Two repos with same directory name in different paths → each appears as separate entry in Projects

[vakovalskii]

LGTM — key-based grouping fixes name collision, breadcrumb + filter reset is nice UX, dataset-based XSS hardening is the right call. Note: overlaps cleanly with #199 changes now that #199 is merged.