feat(richtext-attachment): Add-menu actions target the document end when unfocused

When the article editor has no caret (not first responder), the Add-menu and
table actions previously applied at offset 0 — a structural slot that touches
no paragraph, so paragraph commands silently no-op'd (Heading did nothing) and
inserts landed at the start.

Now, when there is no cursor, these actions target the END of the document:
capture focus state at menu-open time, and before applying, focus the editor
and drop the caret at the end (`focusEditorAtDocumentEndIfNeeded`). Wired into
Text / Quote / Pull-quote / Code / Formula and table insertion. Heading gets a
dedicated path: with a caret it converts the current paragraph as before; with
no caret it appends a NEW empty paragraph in that heading style and focuses it.

Adds `RichTextEditorView.moveCaretToDocumentEnd()`, which collapses the
selection to a caret at `endOfDocument` (past the closing structural token).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>