feat: Object Editor Preview Mode & Mobile Optimizations

Major Enhancements:
- Added Preview/Edit mode toggle to StructuredEditor component
  * Preview mode: Read-only view with full text visibility
  * Edit mode: Full editing capabilities with all controls
  * Toggle positioned below title, responsive on mobile
  * Works in both main ObjectEditor view and nested modals

- Clickable nested data in Preview mode
  * JSON/serialized values are blue and clickable
  * Opens modal directly without switching to Edit mode
  * Hover effects and tooltips for better UX
  * No longer need edit mode just to explore structure

Mobile Responsiveness Improvements:
- Fixed all data load notices in ObjectEditor (URL, Paste, Open tabs)
- Fixed all data load notices in TableEditor (URL, Paste, Open tabs)
- Notices now stack vertically on mobile with proper spacing
- Added break-words for long text, whitespace-nowrap for buttons
- Dark mode colors added for better visibility

Table Editor Fixes:
- Fixed sticky header showing row underneath (top-[-1px])
- Made Export section header mobile responsive
- Updated object modal footer layout:
  * Format info and properties combined on single line
  * Buttons moved to separate row below
  * Changed 'Apply Changes' to 'Save Changes' for consistency

StructuredEditor Improvements:
- Moved overflow-x handling from ObjectEditor to StructuredEditor
- Now works consistently in main view and nested modals
- Long strings scroll horizontally everywhere
- 'Add Property' button hidden in Preview mode
- Improved chevron colors for dark mode visibility

Technical Changes:
- StructuredEditor now manages its own editMode state
- readOnly prop can still be passed from parent if needed
- Proper conditional rendering for all UI elements
- Consistent mobile-first responsive design patterns

src/pages/InvoiceEditor.js

@@ -844,7 +844,7 @@ const InvoiceEditor = () => {
                         type="url"
                         value={url}
                         onChange={(e) => setUrl(e.target.value)}
-                        placeholder="https://api.telegram.org/bot<token>/getMe"
+                        placeholder="https://your-url.com/invoice/your-invoice"
                         className="tool-input w-full"
                         onKeyPress={(e) => e.key === 'Enter' && handleUrlFetch()}
                       />
@@ -858,7 +858,7 @@ const InvoiceEditor = () => {
                     </button>
                   </div>
                   <p className="text-xs text-gray-500 dark:text-gray-400">
-                    Enter any URL that returns JSON data. Examples: Telegram Bot API, JSONPlaceholder, GitHub API, etc.
+                    Enter any URL that returns exported JSON data from your previous invoice work.
                   </p>
                 </div>
               )}