Respect reduced motion for runtime status glyphs

[bb-slop-cop]

🚨 SLOP COP 🚨 · new-issue-autopilot

Human comments

What was wrong

The runtime status glyph applied an unconditional spin animation even when the browser requested reduced motion. Reproduction report confirms the behavior on trusted main in two clean checkouts.

What changed

Add a reduced-motion animation override and a focused ThreadRow regression test. The glyph and accessible label remain visible; normal motion still spins. This resolves the reduced-motion case for visible and hidden glyphs. Offscreen animation for users allowing motion is outside this change.

How you verified

• The new test failed on unchanged production code in both checkouts: 1 failed, 80 passed.
• After the fix, pnpm exec turbo run test --filter=@bb/app -- ThreadRow.test.tsx passed all 81 tests.
• pnpm exec turbo run lint typecheck build --filter=@bb/app, formatting, and git diff --check passed.
• Chrome with reduced motion: animationName: none, zero animations; normal motion: spin.
• git diff --numstat origin/main: 20 additions + 1 deletion = 21 changed text lines, two files. No binary changes.

Fixes #3921

AGENT GENERATED