.composer-bar's two rules leave src/web/studio/feed_style.ts and its name leaves docs/DESIGN.md §5d #4386

Replaces #4245. The class is used nowhere in the markup. Its only uses left are lines 20–21 of src/web/studio/feed_style.ts.

Done:

• [ ] Both .composer-bar rules are deleted, and the name is removed from the studio/feed_style.ts bullet in docs/DESIGN.md §5d. git grep composer-bar finds nothing.
• [ ] Run bun scripts/design-lint.ts --update. Commit docs/design/baseline.json only if the count goes down or stays equal.
• [ ] bun x tsc --noEmit and bun test test/design-lint.test.ts pass.
• [ ] Change nothing else. Both earlier tries failed on unrelated tests (the drain page's hourly cap, webhook refusals). If one fails again, name it in the pull rather than editing it.