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-tauri/Cargo.toml

@@ -28,4 +28,7 @@ tokio = { version = "1", features = ["full"] }
 uuid = { version = "1", features = ["v4"] }
 futures = "0.3"
 bytes = "1"
+png = "0.17"
+base64 = "0.21"
+image = "0.24"