feat: add chat history import and export functionality

Implemented full chat history import/export with JSON format:
- Export button saves current conversation to JSON file
- Import button loads conversation from JSON file
- File dialog integration using tauri-plugin-dialog
- Message count feedback on successful import
- Automatic history reload after import
- Preserves all message data including swipes and timestamps
- Smart error handling (ignores cancelled dialogs)

Backend (Rust):
- export_chat_history: Opens save dialog, writes JSON to selected path
- import_chat_history: Opens file picker, parses JSON, saves to current character
- Message migration for backward compatibility
- Returns helpful feedback (file path on export, message count on import)

Frontend (JavaScript):
- Export/import buttons in header with up/down arrow icons
- Status updates during operations
- Auto-reload chat view after import
- Error handling with user-friendly messages

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/index.html

@@ -30,6 +30,18 @@
             </button>
           </div>
           <div class="header-controls">
+            <button id="import-chat-btn" class="icon-btn" title="Import conversation">
+              <svg width="16" height="16" viewBox="0 0 16 16" fill="none">
+                <path d="M8 11V3M5 8l3 3 3-3" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+                <path d="M3 13h10" stroke="currentColor" stroke-width="1.5" stroke-linecap="round"/>
+              </svg>
+            </button>
+            <button id="export-chat-btn" class="icon-btn" title="Export conversation">
+              <svg width="16" height="16" viewBox="0 0 16 16" fill="none">
+                <path d="M8 3v8M5 6l3-3 3 3" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+                <path d="M3 13h10" stroke="currentColor" stroke-width="1.5" stroke-linecap="round"/>
+              </svg>
+            </button>
             <button id="clear-btn" class="icon-btn" title="Clear conversation">
               <svg width="16" height="16" viewBox="0 0 16 16" fill="none">
                 <path d="M3 4h10M6 4V3a1 1 0 0 1 1-1h2a1 1 0 0 1 1 1v1M5 4v8a1 1 0 0 0 1 1h4a1 1 0 0 0 1-1V4" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" fill="none"/>