Add import UX frontend: source card grid, guided flows, and enhanced history
Replace the import app's generic upload form with a source-aware experience:
- Source card grid as landing page with all 9 import sources
- Per-source guided flow with export instructions (via marked.js) and
source-appropriate upload UI
- Enhanced progress display with accumulating stats and human-readable stages
- Quick Import card preserving the existing drag-and-drop + paste flow
- Import history table with source type column, stats column, and filtering
- Backend: pass --source/--force flags through /api/start, add emoji to
SOURCE_METADATA, expose enriched fields from imported.json via build_import_info