Restore backend/logic features that were incorrectly auto-merged from the
design branch:
- Spreadsheet viewer (xlsx, csv, ods, etc.) and xlsx dependency
- HTML iframe viewer with source toggle
- Directory picker modal for workspace creation
- Workspace registry for custom-path workspaces
- Session auto-discovery for orphaned sessions
- Workspace init seeding (CRM objects, DuckDB, templates, bootstrap files)
- Symlink resolution and showHidden in tree/browse routes
- Upload to workspace assets/ instead of hidden ~/.ironclaw/uploads/
- Webpack dev watcher config (next.config.ts)
- router.push for back-button navigation history
