Fix feed card scroll jumps on expand

[disnet]

Summary

• preserve the tapped feed card position when selecting or expanding collapses a taller card above it
• suppress native scroll anchoring during explicit compensation and share collapse re-anchoring across Less/header toggles
• add deterministic mobile Chromium coverage for collapse-above, body hydration, Less, and deselect paths

Checks

• cd frontend && npm run check (passes: 0 errors, 25 existing warnings)
• frontend/node_modules/.bin/prettier --check e2e/expand-scroll.spec.ts frontend/src/lib/components/feed/FeedListView.svelte
• git diff --check
• node node_modules/playwright/cli.js test e2e/expand-scroll.spec.ts --list (3 tests discovered)
• Browser execution attempted; runner blocks downloaded Chromium with EACCES due no-exec filesystem policy

Radial artifact