feat: comment on a block of a page

[AmrMsCLL]

Notes step N6. Contains a migration — waiting for Amr's approval before merging.

Migration 20261009000000_comment_blocks:

ALTER TABLE "note_comments" ADD COLUMN "block_id" TEXT;

A nullable column with no default, so existing comments are unchanged (they stay page comments).

What it does

• Every block in a page gets a stable id (@tiptap/extension-unique-id, approved; ids are not generated for remote changes, so collaborators don't race).
• The block handle menu gains Comment: the comments box opens quoting that block, and the comment is saved with blockId. Replies stay in the thread and never carry a block id.
• Blocks with open comments get a gold mark in the margin. A block comment shows the block's text; clicking it scrolls to the block and flashes it. If the block was deleted it says so.
• The page format goes to 4, so older tabs reload before ids are added.

Tests: new block comment case in test/notes.spec.tsx, test/block-menu.spec.tsx, lint and type check. test/integration/note-comments.spec.ts covers saving block ids and keeping replies in the thread (CI). Not checked in the browser: that needs the migration applied to the local database, which also waits for approval.