Extract chatStore.ts WebSocket handlers into domain modules

[pufit]

Summary

• Extract the 600-line handleWSMessage switch from chatStore.ts (1307→484 lines) into 4 domain-specific handler modules + 2 helper modules
• Eliminate all 18 as any casts via proper Extract<WSMessage, { type }> discriminated union narrowing
• Deduplicate done/stopped panel cleanup into shared finalizeRunningPanels()
• No component import changes — purely internal reorganization

New file structure

web/src/stores/
├── chatStore.ts              # Thin: state + actions + 30-line dispatcher
├── handlers/
│   ├── streamingHandlers.ts  # thinking, token, tool_use, tool_result, done, stopped, error
│   ├── sessionHandlers.ts    # session_updated/status/switched/forked/resumed/archived/running, answer_injected
│   ├── panelHandlers.ts      # plan_update, subagent_start, subagent_complete, hoa_progress
│   └── auxiliaryHandlers.ts  # interaction, file_changed, notification(s), background_tasks_update
├── helpers/
│   ├── blockHelpers.ts       # appendBlockToPanel, updateToolResultInPanel, auto-close timers
│   └── bufferReplay.ts       # applyStreamEvent, rebuildPanelTabsFromBuffer, deriveStatus, extractTodos

Test plan

• npm run build passes (tsc + vite)
• Streaming works — send a message, tokens flow in real-time
• Session switch restores buffered streaming state
• Sub-agent panel opens/completes/auto-closes
• Stop button works, notifications popup, file changes indicator updates

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