[Raing5Days]

Closes #3738

What this adds

/model is now a composer slash command on the Desktop (Web UI) surface. It was
the one common trigger OpenBitFun did not answer, so switching models always
meant leaving the keyboard.

• Typing /model (or picking it from the slash menu) and pressing Enter clears
  the command text and opens the model selector beside the composer, landing on
  the same view, panel, and focus the trigger opens.
• The menu is opened as a keyboard interaction: focus goes to the current
  choice, and when the menu closes after a selection or dismissal focus returns
  to the composer, so the next message can be typed without reaching for the
  mouse. Mouse-opened menus are unchanged, and jumping to the model settings
  page does not pull focus back.
• The command shares the selector's availability contract. When the selector
  refuses changes (inherited side-session settings, a non-editable draft, a
  locked dispatch submission) it warns and keeps the text instead of silently
  doing nothing. /model <extra> reports command usage.
• /model is offered for local sessions, detached-dispatch sessions (over the
  target model probe), and sessions without thread goals. ACP sessions are
  deliberately untouched: their slash text still goes to the agent, which may
  own its own /model.

Implementation notes

• ModelSelector owns its menu state, so the composer asks it to open through an
  openRequest counter (the same pattern as FlowChatHeader.searchOpenRequest)
  and receives onRequestMenuClosed for the focus handoff. No prop controls the
  open state directly, and the trigger path is unchanged.
• The composer derives a single modelSelectorDisabled value and uses it for
  both the selector and the command, so the two cannot disagree.
• Slash command surface: ComposerSlashOp gains model for the local, non-goal,
  and dispatch capability sets; new strings in en-US / zh-CN / zh-TW.

Verification

• pnpm --dir src/web-ui run test:run over 8 focused files, all green:
  • new ModelSelectorOpenRequest.test.tsx: opens on request, answers each
    request value once, ignores a request while disabled, hands focus back only
    for requested menus;
  • useComposerCapabilities.test.tsx: local and detached sessions both
    advertise model;
  • existing ModelSelector / composer layout suites unchanged.
• pnpm run type-check:web
• pnpm run i18n:audit
• pnpm run check:repo-hygiene
• eslint over the touched sources

Remote scenarios: no transport, workspace path, or persisted-shape changes. The
command is renderer-local and reuses each driver's existing model selection
(local catalog, ACP options, detached target probe), so remote paths keep their
current owners. Not exercised against live remote hosts.

Follow-up (not in this PR)

• CLI alignment: the CLI command menu
  (src/apps/cli/src/ui/command_menu.rs) keeps its own command set; this PR does
  not touch it. Adding /model there, and any shared command-catalog contract
  between the surfaces, is left as a separate change.
• The command takes no arguments for now; a /model <name> query could build on
  the same request signal later.