Add font roundtrip export and quick glyph creation

[kostyafarber]

Summary

• add the Rust-backed font export path, including desktop menu/IPC wiring for TTF export
• cover MutatorSans UFO -> TTF -> readback round trips in tests
• add sidebar quick glyph creation with auto-incrementing newGlyph names
• remove the modal-based new glyph flow from the final branch shape

Validation

• pre-commit run during commit, including cargo check/fmt/clippy/test, oxfmt, oxlint, tsgo typecheck, deadcode strict, and vitest
• pnpm --filter @shift/desktop exec vitest run src/lib/model/Font.test.ts
• pnpm --filter @shift/desktop typecheck
• pnpm --filter @shift/desktop lint:check