fix(desktop): isolate full composer state per tab (#4133)

[JesonChou]

Problem

Fixes #4133. When multiple tabs are open, the composer input box content was shared across all tabs instead of being independent. Editing text (or attaching files, adding @-refs, pasting blocks, selecting past:chats sessions) in Tab A was visible
in Tab B and vice versa.

Root cause: The <Composer> component was a single instance without a key prop, so React preserved its local state across tab switches. The per-tab State map (TabStates) did not track any composer draft state.

Solution

Three-part fix:

1. Per-tab draft storage — State interface gains a draft: ComposerDraft field that persists all user-facing composer state (text, attachments, workspace @-refs, pasted blocks, session refs) keyed by tab ID.

2. Force remount on tab switch — <Composer key={activeTabId}> ensures React unmounts the old tab's composer and mounts a fresh instance for the new tab.

3. Save on unmount, restore on mount — A useEffect cleanup saves the full ComposerDraft to per-tab state when the component unmounts. useState initial values read from the draft prop on mount. A reference-equality guard skips the save when nothing changed, avoiding wasted re-renders.

Design decisions

Decision	Rationale
key={activeTabId} instead of useEffect on tabId	Guarantees clean isolation — no state leaks between tabs. The saved draft is the only bridge.
ComposerDraft aggregate type	Single field is simpler than 5 separate draft fields; setDraft is one dispatch call per unmount.
Reference-equality guard in cleanup	cur.text === draft.text && cur.attachments === draft.attachments … — since every state is initialised from draft.*, untouched arrays share the same reference. Skipping the save avoids setDraft → bump() → re-render.
UI-only state (menus, drag, height) not persisted	These are transient and cheap to reset; composerHeight already has its own layout-preference persistence.
setDraft passed directly (not inline arrow)	useCallback-wrapped → stable reference → no effect re-trigger loop.

Files changed

File	+/−	Notes
desktop/frontend/src/lib/types.ts	+28	New Attachment, WorkspaceReference, PastedBlock, ComposerDraft types
desktop/frontend/src/lib/useController.ts	+15/−1	draft field, set_draft action, setDraft() export, reset guard
desktop/frontend/src/components/Composer.tsx	+60/−23	Import types from types.ts, restore/save full draft, skip-noop guard
desktop/frontend/src/App.tsx	+4	key={activeTabId}, draft={state.draft}, onDraftChange={setDraft}

Verification

• ✅ 190 frontend unit tests pass (zero failures)
• ✅ TypeScript compiles cleanly (tsc --noEmit)
• ✅ Manually tested on Windows: multiple tabs each retain independent composer state (text, attachments, @-refs, pasted blocks) across tab switches