- Main store: replace useReducer with Zustand; pure state transition
  functions (syncServerReadModel, markThreadVisited, setError, etc.)
  called via store methods. Persist via subscribe.
- Terminal store: single store keyed by threadId; replace
  dispatchTerminalAction with direct methods (setTerminalOpen,
  setTerminalHeight, splitTerminal, newTerminal, setActiveTerminal,
  closeTerminal, setTerminalActivity). Pure reduceThreadTerminalState
  used internally.
- Update all consumers to call dispatch.method(...) and terminal
  store methods. Fix store tests to use markThreadUnread pure fn.
- Fix Fragment import in store.ts; fix unused get param for lint.

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