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

.claude/settings.local.json

@@ -9,7 +9,12 @@
       "Bash(WAYLAND_DISPLAY=\"\" DISPLAY=:0 ./src-tauri/target/debug/tauri-app:*)",
       "Bash(ldd:*)",
       "Read(//home/matt/.config/claudia/**)",
-      "WebFetch(domain:github.com)"
+      "WebFetch(domain:github.com)",
+      "Bash(cat:*)",
+      "Bash(npm run tauri:dev:*)",
+      "Bash(git add:*)",
+      "Bash(git commit:*)",
+      "WebSearch"
     ],
     "deny": [],
     "ask": []