- Added CharacterCardV3 struct to handle v3 format
- Created From<CharacterCardV3> for CharacterCardV2Data conversion
- Updated read_character_card_from_png() to detect and parse both v2 and v3 specs
- V3 cards have top-level fields + nested data object
- Maintains full backward compatibility with v2 cards
- Export still uses v2 format for maximum compatibility
- 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
