fix(auth): clear the mobile feedback FAB from the OAuth sign-in row

[catomean]

Summary

• Visitor feedback on https://orangecat.ch/auth?mode=login&from=%2Fmessages (where every logged-out /messages visitor lands): at 320×800, the Sign in with GitHub button's bottom-right corner sits under the fixed FleetCrown feedback FAB (#fleetcrown-feedback-host, bottom:16 right:16, 48px, max z-index) — the FAB steals the tap. Cost: signups on the sign-in page.
• The widget itself is an external embed (fleetcrown.orangecat.ch/widget.js, loaded once in layout.tsx) — not in this repo, so its own positioning isn't something this PR can change or verify.
• Root cause on our side: below ~360px, the heading text wraps onto more lines, making the page ~45–70px taller and pushing the OAuth button row into the FAB's fixed screen-corner on the very first, unscrolled paint — reproduced exactly: visitor measured the GitHub button at x166-288/y701-745; a Playwright repro against the real page (with /api/auth/oauth-providers mocked to render the OAuth row, since this sandbox has no live provider config) landed it at the identical x166-288/y701-745, overlapping the FAB's y736-784 band by 9px.
• Fix: trimmed non-essential vertical rhythm above the OAuth row — heading margin, anonymous-CTA block margin, outer top padding, OAuth-block top margin — scoped to max-[359px]: only (an arbitrary Tailwind variant, no tailwind.config.ts change). No touch-target sizes changed. Viewports ≥360px are untouched (and were never affected — the overlap is a narrow-viewport-only artifact of text reflow).

Test plan

• npx tsc --noEmit — clean
• npx eslint src/app/auth/page.tsx src/app/auth/AuthSocialLogin.tsx — clean
• Full local npm run test:unit (2638 tests) and full pre-push suite — green
• Playwright measurement against the real running page at 320×800 (OAuth-providers API mocked to render Google/GitHub, since no live provider config exists in this sandbox): GitHub button before fix at x166-288/y701-745 (9px vertical overlap with the FAB's y736-784 reserved corner) → after fix at x166-288/y645-689, 47px clear.
• Re-checked at 320/359/360/375/390/430px: clear at every width; 360px+ renders byte-for-byte the same as before this change (the compaction only applies under 360px) and was already clear pre-fix, confirming no regression on normal phone widths.
• Could not load the real widget script (no network path to fleetcrown.orangecat.ch + no FLEETCROWN_FEEDBACK_TOKEN in this sandbox) or real OAuth provider config — verification used a same-geometry synthetic overlay for the FAB and a mocked provider-list response; both are documented above with exact before/after coordinates so the numbers are checkable against the live site.

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