feat(review-tutor): show a pulsing ember dot while the tutor answers

[ElbertePlinio]

Refs #64 (human smoke feedback on #68). Stacked on #67; part of the batched Review Tutor chain.

What changed

• The busy Ask button shows one cue: a pulsing ember dot beside Sending → Queued → Answering, replacing the bare "Running" text. Nothing else changes; the answer block still appears only once content exists.
• Reduced motion: the dot renders solid. The dot is aria-hidden; announcements and the button's accessible name are unchanged.

Tested

• Page tests for the label sequence and dot presence only while busy; bun run check green.
• Visual acceptance in Elberte's live smoke (the first version with an extra progress line and early Answer block was rejected as overdesign and removed).

Human gate: merge only on Elberte's authorization.