refactor(ui): decompose composer and plan-reminder panels into focused hooks

[Astro-Han]

Context

Two shared-UI panels fold several independent state machines into one component.

• composer.tsx (1124): draft persistence (draftStoreRef, saveCurrentDraft, swap-effect
  323-341) + prompt-history nav (481-526) + @/ mention state machine (270-280, 550-580,
  610-667) + drag/drop+paste import (669-737) + toolbars (822-1114).
• plan-reminder-panel.tsx (863): ~20 useState mixing edit-form fields (127-134), list
  filter/sort/search (145-148), run-history range, dialog + per-action pending; the
  create/edit dialog form (668-860, ~190 JSX lines) is self-contained.

Proposed

• Composer: extract useComposerDraft, useComposerHistory, useMentionPopup (or a
  child), and a ; composer orchestrates only.
• Plan-reminder: extract owning all form state + submit; the
  panel keeps list/runs/query state.

Acceptance

Each hook/component owns one concern; the panels no longer hold every field.