perf(auth): static gradient + shared one-shot orbs across all auth pages

[ahmetabdullahgultekin]

What & why

PR #217 (c8c017d) removed the login-page jank anti-pattern from LoginPage. The 7 other auth shells each carried a byte-identical copy of it, so the same jank persists on register / MFA / verify-email / reset / onboarding / accept-invite / forgot-password. This PR fixes all 7 and eliminates the resulting DRY violation.

The anti-pattern (per page)

1. The brand gradient was painted with an infinite gradientShift keyframe over backgroundSize: '400% 400%'. Animating background-position cannot be GPU-composited → a full-viewport repaint every frame (on the main thread when hardware-acceleration is off).
2. Decorative FloatingShape orbs each ran an INFINITE framer-motion loop with a per-frame backdrop-filter: blur(10px) — continuous main-thread compositing cost.

The fix (presentational / perf only — behaviour-preserving)

• Replace every inline animated-gradient sx block with the shared loginShellBackgroundSx() (static brand gradient, identical stops) from loginBackground.ts; the dead @keyframes gradientShift are removed.
• DRY extraction: new shared component src/features/auth/components/FloatingShape.tsx — the cheap one-shot fade/scale-in (opacity 0→0.28, scale 0.8→1, duration 1.2, ease easeOut) with a soft radial fill and no animated backdrop-blur (exactly the version LoginPage already shipped in perf(login): static shell gradient — fix general dashboard login jank #217). LoginPage and all 7 pages now import it; their local inline FloatingShape definitions are deleted (8 duplicate copies → 1).
• Orbs are gated behind !usePrefersReducedMotion() on every page that renders them, matching LoginPage.

Each page keeps its exact colours, layout, glass-card backdrop-filter: blur(20px) (cheap over a now-static gradient; none of these pages has a live camera step), and orb positions/sizes/delays.

Reversibility

Pure presentational change. The static-gradient builder lives in loginBackground.ts (restore the gradientShift animation there to revert); the orb component is a single file.

Orb counts preserved (proof)

Page	Orbs
LoginPage (re-uses shared component, output unchanged)	5
RegisterPage	5
SecondaryAuthFlow	0 (had an unused local FloatingShape def + the animated gradient — both removed; renders no orbs)
VerifyEmailPage	4
ResetPasswordPage	5
OnboardingPage	5
AcceptInvitePage	5
ForgotPasswordPage	5

Files changed

• new src/features/auth/components/FloatingShape.tsx (shared one-shot orb)
• src/features/auth/components/LoginPage.tsx (migrated to shared orb; output identical)
• src/features/auth/components/RegisterPage.tsx
• src/features/auth/components/SecondaryAuthFlow.tsx
• src/pages/VerifyEmailPage.tsx
• src/pages/ResetPasswordPage.tsx
• src/pages/OnboardingPage.tsx
• src/pages/AcceptInvitePage.tsx
• src/pages/ForgotPasswordPage.tsx

Net -196 LOC across the 8 files + the one shared component.

Verification

Check	Result
npx tsc --noEmit	✅ exit 0, no errors
npx eslint <9 changed files>	✅ exit 0, no warnings
npx vitest run src/features/auth src/pages	✅ 395/395 tests in 52 files
npx vite build	✅ exit 0 (pre-existing chunk-size + onnxruntime eval warnings only)