fix(frontend): make auth panel vortex animation frame-rate independent (#13194)

### Why / What / How

**Why:** The vortex particle animation on the login/signup left panel
was tied to frame rate. Higher-refresh displays (120/144/240Hz) ran the
animation up to ~2.4× faster than 60Hz, and slower CPUs made it crawl
unevenly. The user reported it "feels tied to CPU speed."

**What:** Make the vortex animation frame-rate independent using delta
time, and slow the overall drift on the auth marketing panel.

**How:**
- `vortex.tsx`: track `lastTime` via `performance.now()` each frame and
compute `dtFrames = dt * 60` (normalized to a 60 fps reference, capped
at 2 frames to avoid jumps after tab inactivity). Scale all per-frame
increments by `dtFrames`:
  - position: `x + vx * speed * dtFrames`
  - noise advance: `tick += dtFrames`
  - life: `life += dtFrames`
- velocity smoothing uses the frame-rate-independent form `1 -
0.5^dtFrames` instead of constant `0.5`
- `AuthMarketingPanel.tsx`: lower `rangeSpeed` from `1.2` to `0.22` and
add `baseSpeed={0.08}` so particles drift slowly (range 0.08–0.30 vs.
0–1.2 before).

Result: identical perceived speed across 60/120/144/240Hz displays and
across CPU tiers, at a calmer overall pace.

### Changes 🏗️

- `src/components/ui/vortex.tsx`: delta-time scaling for `tick`,
particle position, life, and velocity smoothing.
- `src/components/auth/AuthSplitLayout/AuthMarketingPanel.tsx`: lower
vortex speed (`baseSpeed=0.08`, `rangeSpeed=0.22`).

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [ ] I have tested my changes according to the test plan:
- [ ] Open `/login` and `/signup` on a 60Hz display — particles drift
slowly and smoothly
- [ ] Open the same pages on a high-refresh display (120/144Hz) —
animation speed matches 60Hz, not visibly faster
- [ ] Switch tabs and return — animation does not "jump" on resume
(delta clamp at 1/30s)
  - [ ] Resize window — particles continue without artifacts

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>