Add persistent plan mode and thread goals

[Qjzn]

What changed

• keep Plan mode enabled until the user explicitly disables it or implements a plan
• render structured plan events as stable expandable cards with progress and persisted implementation state
• add authoritative thread-level goals with create, edit, pause, resume, clear, progress, refresh recovery, and idle continuation
• isolate goal editor, menu, and confirmation state across thread switches
• improve phone ergonomics, keyboard focus behavior, destructive confirmation, and Plan + Goal coexistence guidance
• add normalization coverage, regression fixtures, and manual acceptance contracts

Why

Plan was previously treated like a one-turn send option, structured plan output could be flattened or hidden, and the frontend did not own the App Server thread-goal lifecycle. This made long-running work difficult to understand and fragile across refreshes, thread switches, and mobile use.

User impact

Users can keep Plan mode active, explicitly implement the latest eligible plan, and manage a persistent goal for each conversation without transient state leaking into another thread. Mobile actions remain reachable without horizontal overflow.

Validation

• npm run build:frontend
• npm run verify:frontend-normalizers
• npm run verify:server-modules
• vue-tsc --noEmit
• git diff --check
• headless Playwright at 393x852 and 1280x800 for Plan/Goal coexistence, cross-thread isolation, keyboard focus, durable clear confirmation, history-derived implementation state, 44px actions, and overflow

The broad live test:7420:frontend run stopped at its idle preflight because the current handling thread was still the single active runtime request; no runtime state was force-cleared.