Align tables to prose and polish unavailable images, formula toggles, and icon titles

- Tables start at the prose edge instead of centring; wide ones scroll within
  the prose measure while the rails stay in the gutter.
- An image that fails to load becomes a quiet framed placeholder when it is
  alone in its paragraph and an inline chip inside a sentence, both labelled
  by the alt text. Adds a Nucleo-style ImageIcon.
- A block formula's show-source toggle sits in its top-end corner and is
  revealed on hover or focus (always shown on touch).
- Icons no longer render <title>, which showed the raw asset name as a
  native tooltip.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>