refactor: trim unused client surface

[KyleDerZweite]

Summary

• Collapses the one-implementation backend trait into MockBackend and removes 13 unused bridge commands plus their stubs.
• Removes unproduced event variants and orphan event types, then regenerates src/bindings.ts.
• Removes the unused async-trait and direct serde_json dependencies plus the unused MockBackend default implementation.
• Reduces TypeScript and Tailwind configuration to settings used by the current client.
• Inlines the default TypeScript binding configuration and updates compatibility documentation.

Skipped findings

• None. All seven audit findings matched current callers, event producers, and configuration use.

Compatibility

• The generated command API now exposes restoreSession, listSpaces, listRooms, loadTimeline, and sendMessage.
• The event union now exposes Timeline and Typing.
• Rust users of ChatBackend must use MockBackend directly. Restore an operation with backend behavior, a Tauri wrapper, and a UI caller. Add a shared trait when a second backend needs it.

Checks

• Rustfmt check using edition 2021 on every changed Rust source
• Cargo tests with workspace scope
• Frontend typecheck and production build with bun run build