fix(toolbar): sync undo and redo availability when controls mount

[huyanxius]

When the documented ConditionalContents toolbar replaces UndoRedo with the code-language selector, returning to a paragraph leaves Undo disabled even though Cmd/Ctrl+Z can undo the inserted code block. Initialize the controls from the active editor's history so toolbar Undo and Redo work as soon as the controls remount.

Why

I reproduced this on current main (122347cff8af5e9e1291b52789d22ed3ba0dc2ba) with React 19.2.1: start with an empty editor, insert a code block, focus its code, then click the paragraph below it. Undo remains disabled; Cmd+Z removes the block and enables Redo. The existing focus-time replay does not resolve this case.

CAN_UNDO_COMMAND and CAN_REDO_COMMAND notify listeners about changes; registering a listener does not replay commands emitted while the component was unmounted. UndoRedo previously initialized both flags to false on every mount.

Changes

• Read the active editor's HistoryExtension state when the effect attaches, respecting disabled or absent history, then retain the existing command subscriptions for subsequent updates.
• Add unit coverage for conditional remounts with Undo and Redo history, empty/suppressed history, and switching between root, local-history, history-free, and absent editors.
• Add a small bug-654 example and a browser regression for the actual code block/paragraph/toolbar undo/redo flow.

Verification

• Regression written and observed failing on unmodified main at the remounted Undo button assertion, before the production change.
• Rechecked head 17fb56702158bc4d277129d34fec89d87a4c7e70: npm run lint and npm run typecheck passed. The full Vitest suite passed with one worker: 126 passed, 1 skipped, 1 todo, including all 15 focused tests across undo-redo.test.tsx, toolbar.test.tsx, and extension-composer.test.tsx.
• Actual Chromium reproduction before/after: before, toolbar Undo stays disabled while Cmd+Z works; after, toolbar Undo removes the code block and Redo restores it. Empty-document Undo/Redo remain disabled. No page errors.
• Library build passed. The packed React 18 consumer's declarations, Vite bundle, and Lexical dependency graph checks passed.
• The checked-in Playwright/Ladle regression and packed React 18 browser runtime remain unverified in the cloud environment: Ladle stops before page creation with uv_interface_addresses, and Chromium startup fails with socket() failed: Operation not permitted. The isolated Chromium reproduction above is separate from the checked-in Playwright test.
• git diff --check: passed.

Fixes #654

Upstream CI for the current head is action_required, awaiting approval to run this fork workflow: CI run. The implementation is ready for review; browser CI still needs to run.