fix(flow-chat): reorganize composer context controls

[bobleer]

Summary

• reorder the composer context rail to show workspace and branch before the execution target
• move worktree isolation into the local target menu as New Worktree, reflecting the selected local mode on the trigger
• remove the down chevrons from the workspace and execution-target controls while preserving their menus
• keep the standalone worktree switch only as a fallback for embedded surfaces without the target picker

Verification

• pnpm --dir src/web-ui run test:run src/features/dispatch/DispatchTargetPicker.test.tsx src/flow_chat/components/ChatInputWorkspaceStrip.test.tsx src/flow_chat/components/ChatInputWorkspaceStripLayout.test.ts (54 tests)
• pnpm run check:web
• pnpm run i18n:audit
• targeted ESLint for changed TypeScript files (0 errors; test files are ignored by repository config)
• browser visual/interaction QA in dark zh-CN state: workspace menu, local menu, New Worktree selection, local restoration, and icon counts

Remote scenarios

This is a Web UI presentation and local session-mode wiring change. It was exercised in a local browser harness. Remote workspace, remote control, Peer Device Mode, and Detached Dispatch behavior are unchanged and were not separately exercised.

AI assistance

Implemented and verified with AI assistance.