feat: add v2 character card import/export

- Expanded Character struct with all v2 spec fields (description, scenario, mes_example, post_history_instructions, alternate_greetings, character_book, tags, creator, character_version, creator_notes, extensions)
- Created CharacterCardV2 serialization structs following spec at github.com/malfoyslastname/character-card-spec-v2
- Implemented PNG metadata utilities:
  * read_character_card_from_png() - extracts and decodes character data from PNG tEXt chunks
  * write_character_card_to_png() - embeds character data into PNG files
  * create_placeholder_png() - generates gradient placeholder images for avatarless characters
- Added Tauri commands:
  * import_character_card - opens file picker, imports PNG with automatic name conflict handling
  * export_character_card - exports character as v2 PNG card with embedded metadata
- Added Import/Export buttons to character settings UI
- Full backward compatibility with existing characters using serde defaults
- Added dependencies: png 0.17, base64 0.21, image 0.24

src/main.js

@@ -971,6 +971,8 @@ function setupAppControls() {
   });
   document.getElementById('upload-avatar-btn').addEventListener('click', handleAvatarUpload);
   document.getElementById('remove-avatar-btn').addEventListener('click', handleAvatarRemove);
+  document.getElementById('import-character-btn').addEventListener('click', handleImportCharacter);
+  document.getElementById('export-character-btn').addEventListener('click', handleExportCharacter);
 }
 
 // Keyboard shortcuts
@@ -1077,6 +1079,51 @@ async function handleDeleteCharacter() {
   }
 }
 
+// Handle character card import
+async function handleImportCharacter() {
+  const characterMsg = document.getElementById('character-message');
+  try {
+    const importedCharacter = await invoke('import_character_card');
+    characterMsg.textContent = `Successfully imported ${importedCharacter.name}!`;
+    characterMsg.className = 'validation-message success';
+
+    // Reload characters and switch to the imported one
+    await loadCharacters();
+    await loadCharacterSettings();
+
+    setTimeout(() => {
+      characterMsg.style.display = 'none';
+    }, 3000);
+  } catch (error) {
+    console.error('Failed to import character:', error);
+    if (error && !error.toString().includes('No file selected') && !error.toString().includes('cancelled')) {
+      characterMsg.textContent = `Failed to import: ${error}`;
+      characterMsg.className = 'validation-message error';
+    }
+  }
+}
+
+// Handle character card export
+async function handleExportCharacter() {
+  const characterMsg = document.getElementById('character-message');
+  try {
+    const characterId = document.getElementById('character-settings-select').value;
+    const outputPath = await invoke('export_character_card', { characterId });
+    characterMsg.textContent = `Successfully exported to ${outputPath}`;
+    characterMsg.className = 'validation-message success';
+
+    setTimeout(() => {
+      characterMsg.style.display = 'none';
+    }, 3000);
+  } catch (error) {
+    console.error('Failed to export character:', error);
+    if (error && !error.toString().includes('cancelled')) {
+      characterMsg.textContent = `Failed to export: ${error}`;
+      characterMsg.className = 'validation-message error';
+    }
+  }
+}
+
 // Load chat history
 async function loadChatHistory() {
   try {