Add GUI /ultracode on/off toggle choices with keyboard selection

[yxlyx]

Problem

Typing /ultracode should not behave like an unknown slash command. It should open an explicit on/off toggle UI so users can flip persistent ultracode mode from the composer.

Desired behavior

• Typing /ultracode opens toggle choices: /ultracode on and /ultracode off.
• The opposite of the current ultracode state is listed and highlighted first.
  • Current off -> on first.
  • Current on -> off first.
• Enter selects the highlighted choice.
• ArrowUp/ArrowDown navigate between choices.
• Selecting a choice dispatches /ultracode on or /ultracode off.
• The selected mode is stored and applies to subsequent prompts.

Implementation notes

• Extend slash autocomplete with an ultracode choice mode.
• Render toggle choice rows in the existing command popup.
• Wire choice selection through the command router.
• Add /ultracode to GUI command descriptors and QA mocks.
• Store frontend state for ordering and backend state for prompt steering.

Verification

• Focused autocomplete/router/component tests pass.
• GUI production build passes.
• Tauri runtime simple tests pass.

[justrach]

@yxlyx Shipped — typing /ultracode now opens an on/off toggle picker with keyboard selection (ArrowUp/Down + Enter, opposite-of-current state highlighted first), commit b3dd7bf, in v0.0.166. Thanks!