Add defaultStyle to a TipTap text block

[VPS-Andreas]

The styling select always offered a "Default" entry standing for "no style", even where a design has no unstyled variant and every paragraph or heading is meant to carry one of the configured styles. Editors had to pick the right style by hand on every new paragraph, and forgetting it produced unstyled content.

A text block (or list) with a defaultStyle has no such state:

createTipTapRichTextBlock({
    textBlocks: [
        { name: "heading-1", label: "Heading 1", tag: "h1", styles: headlineStyles, defaultStyle: "headline300" },
        { name: "heading-2", label: "Heading 2", tag: "h2", styles: headlineStyles, defaultStyle: "headline400" },
    ],
});

The select drops its "Default" entry, and the style is applied to new content and to every paragraph or heading the editor creates without one — pressing Enter at the end of a heading, the Mod-Alt-<level> shortcuts, pasting. Rather than reimplementing each of those paths, a ProseMirror plugin fills in a missing style on document change, so opening older content leaves it untouched and does not mark the document as changed. Switching the type keeps a style the new text block also offers and falls back to its defaultStyle otherwise, and toggling a list hands the paragraph to the list's styles or back — through the toolbar buttons and Mod-Shift-7/Mod-Shift-8 alike.

Because the default sits on the text block rather than on a tag, two text blocks sharing a tag can have different defaults, and a text block without one keeps its "Default" entry next to text blocks that have one. buildApplyTextBlocksMigration gives existing content its style up front and replaces a style a text block no longer offers.

Third of four stacked pull requests, on top of #6383 and #6382; #6385 splits the block's Storybook stories by topic.

Example

createTipTapRichTextBlock.test.ts/.test.tsx cover the validation and the migration. The Storybook story Default Text Block Style walks through a text block with a default, one without, and the styled lists.

Session: https://claude.ai/code/session_f3c6ad37-c68b-4187-85b6-2abe5aa95c35

[VPS-Obi]

We'll review this once #6383 has been merged.