feat(chat): unify cross-backend model switching

[kessenma]

Summary

• unify the chat model picker across Claude, Codex, and OpenCode with backend-grouped options
• infer and persist session backend from the selected model so users can switch models mid-session
• update model labels and tests to use shared backend-aware model metadata

Testing

• bun run typecheck
• bun run vitest run src/components/chat/hooks/session-setting-sync.test.ts src/components/chat/toolbar/toolbar-options.test.ts
• bun run eslint src/components/chat/ChatToolbar.tsx src/components/chat/QueuedMessageItem.tsx src/components/chat/approval-label-utils.ts src/components/chat/hooks/session-setting-sync.ts src/components/chat/hooks/session-setting-sync.test.ts src/components/chat/hooks/useToolbarHandlers.ts src/components/chat/toolbar/DesktopToolbarControls.tsx src/components/chat/toolbar/MobileToolbarMenu.tsx src/components/chat/toolbar/toolbar-options.ts src/components/chat/toolbar/toolbar-options.test.ts src/components/chat/toolbar/types.ts src/components/chat/toolbar/useToolbarDerivedState.ts src/lib/model-utils.ts

current:

this branch: