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/index.html

@@ -191,7 +191,20 @@
             </button>
             <button type="button" id="delete-character-btn" class="btn-danger">
               Delete Character
-            </button>          </form>
+            </button>
+
+            <div class="form-group" style="margin-top: 1rem; padding-top: 1rem; border-top: 1px solid var(--border-color);">
+              <label>Character Card Import/Export</label>
+              <div style="display: flex; gap: 0.5rem;">
+                <button type="button" id="import-character-btn" class="btn-secondary" style="flex: 1;">
+                  Import v2 Card
+                </button>
+                <button type="button" id="export-character-btn" class="btn-secondary" style="flex: 1;">
+                  Export v2 Card
+                </button>
+              </div>
+            </div>
+          </form>
         </div>
       </div>