feat: implement professional modal editing forms (Phase 1.2)

MAJOR UX IMPROVEMENT: Replace basic prompt() with professional forms

New Features:
- Professional modal overlays with backdrop and ESC/click-outside cancel
- Dynamic form generation based on content type and HTML element
- Smart field detection: H1-H6→text, P→textarea, A→link with URL
- Mobile-responsive form positioning and widths
- Complete CSS styling with focus states and transitions
- Proper save/cancel event handling

Technical Implementation:
- Created lib/src/ui/form-renderer.js with modern ES6+ modules
- Integrated into core editor.js with form renderer instance
- Support for text, textarea, markdown, and link field types
- XSS protection with HTML escaping
- Responsive design: mobile-first form sizing
- Professional styling matching prototype quality

Before: Basic browser prompt() for all editing
After: Content-aware professional modal forms

This brings the library from proof-of-concept to professional-grade
editing experience, closing the major UX gap with the archived prototype.

Phase 1.2 ✅ COMPLETED - Next: Authentication system (Phase 1.1)

TODO.md

@@ -31,11 +31,20 @@ Bring the current library (`lib/`) up to feature parity with the archived protot
 - [ ] Create authentication controls (login/logout toggle)
 - [ ] Add edit mode toggle (separate from authentication)
 
-#### 1.2 Professional Edit Forms ⭐ **HIGH IMPACT**
-- [ ] Replace prompt() with professional modal overlays
-- [ ] Create dynamic form renderer based on content type
-- [ ] Implement smart form positioning relative to elements
-- [ ] Add mobile-responsive form layouts
+#### 1.2 Professional Edit Forms ⭐ **HIGH IMPACT** ✅ **COMPLETED**
+- [x] Replace prompt() with professional modal overlays
+- [x] Create dynamic form renderer based on content type
+- [x] Implement smart form positioning relative to elements
+- [x] Add mobile-responsive form layouts
+
+**Implementation Details:**
+- Created `lib/src/ui/form-renderer.js` with modern ES6+ modules
+- Professional modal overlays with backdrop and ESC/click-outside to cancel
+- Dynamic form generation: text, textarea, markdown, link (with URL field)
+- Smart field detection based on HTML element type (H1-H6, P, A, etc.)
+- Responsive positioning and mobile-optimized form widths
+- Complete CSS styling with focus states and transitions
+- Integrated into main editor with proper save/cancel handlers
 
 #### 1.3 Content Type Support
 - [ ] Text fields with length validation