feat: add HEIC image display support

[tacogips]

Summary

• classify HEIC/HEIF files as image previews
• resolve Markdown HEIC/HEIF image assets through the local preview pipeline
• add fallback UI for image decode/load failures

Verification

• bun run typecheck
• bun run test
• bun run test:dom
• CARGO_TERM_QUIET=true cargo check
• CARGO_TERM_QUIET=true cargo test
• CARGO_TERM_QUIET=true cargo clippy --all-targets --all-features
• bun run tauri build --debug --no-bundle