feat(desktop): replace PNG persona export with JSON format

[wesbillman]

Summary

Replace the PNG-based persona export/import format with a clean JSON format for better compatibility with other tools when sharing persona files.

Exported .persona.json format

{
  "version": 1,
  "displayName": "Researcher",
  "systemPrompt": "You are a research agent...",
  "avatarUrl": "https://example.com/avatar.png"
}

The avatarUrl field supports three scenarios:

• Regular URL: "https://example.com/avatar.png"
• Base64 data URI: "data:image/png;base64,iVBORw0KGgo..." (inline image, round-trips through export → import)
• No avatar: field is omitted entirely

What changed

Export: Personas now export as .persona.json files instead of .persona.png. Avatars (including base64 data URIs) are preserved in the avatarUrl field.

Import: Accepts .persona.json, .persona.png (backwards compat), and .zip (containing either format). JSON import reads avatarUrl and populates the avatar on the imported persona.

Removed: PNG encoding pipeline (encode_persona_png, generate_placeholder_png, hsl_to_rgb, chara card encoding). PNG decoding kept for backwards-compatible import of old .persona.png files.

Changes (9 files, +232/-249)

Rust backend:

• persona_card.rs: Added parse_json_persona() (with avatarUrl support), encode_persona_json(), extracted shared extract_sprout_fields() helper, updated ZIP parser for .json entries, removed PNG encoding functions
• commands/personas.rs: Replaced export_persona_to_png with export_persona_to_json, updated parse_persona_files for JSON detection
• lib.rs: Updated command registration

TypeScript frontend:

• Single "Export" button (JSON only) replaces "Export PNG"
• Drag-and-drop accepts .persona.json, .persona.png, and .zip
• Updated API layer, hooks, and test harness

Testing

• 49 unit tests pass (8 new JSON-specific tests, including avatar round-trip scenarios)
• Full CI green: fmt, clippy, desktop-check, desktop-build, rust-tests
• Backwards compat: old .persona.png files still importable