feat: Implement Tailwind CSS UI redesign across application

Migrate frontend from custom CSS to Tailwind CSS framework with comprehensive
template updates and improved component structure.

Breaking Changes:
- Remove legacy CSS files (base.css, calendar.css, ui.css, etc.)
- Replace with Tailwind-based styling system

New Features:
- Add Tailwind CSS configuration with PostCSS pipeline
- Create new template components for admin, clients, invoices, projects, reports
- Add form-bridge.css for smooth transition between legacy and Tailwind styles
- Add default avatar SVG asset
- Implement Tailwind-based kanban board template
- Add comprehensive UI quick wins documentation

Infrastructure:
- Add package.json with Tailwind dependencies
- Configure PostCSS and Tailwind build pipeline
- Update .gitignore for Node modules and build artifacts

Testing:
- Add template rendering tests (test_tasks_templates.py)
- Add UI component tests (test_ui_quick_wins.py)

Templates Added:
- Admin: dashboard, settings, system info, user management
- Clients: list and detail views
- Invoices: full CRUD templates with payment recording
- Projects: list, detail, and Tailwind kanban views
- Reports: comprehensive reporting templates
- Timer: manual entry interface

This commit represents the first phase of the UI redesign initiative,
maintaining backward compatibility where needed while establishing the
foundation for modern, responsive interfaces.

.gitignore

@@ -185,4 +185,11 @@ nginx/ssl/*.crt
 # docker-compose.https.yml is now tracked
 
 # Environment backups
-.env.backup
+.env.backup
+
+# Node.js / Frontend build
+node_modules/
+package-lock.json
+
+# Tailwind CSS build output (keep source in git)
+app/static/dist/