fix(dashboard): onboarding step buttons + tighter auth provider spacing

- ModelOnboardingModal: completed/skipped step indicators render as
  <button> so they're clickable for review. Without explicit reset,
  browsers gave them their default white-ish background. Added
  background: transparent + border/padding/font-family resets to
  .model-onboarding-step-indicator so it inherits cleanly from the
  surrounding card on every theme.
- SettingsModal authentication: tightened spacing around providers —
  group margin 16→10, group label margin 8→4, provider card margin
  8→4, header padding 12 16 → 8 12. Less wasted space between rows.

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

packages/dashboard/app/components/ModelOnboardingModal.css

@@ -79,6 +79,13 @@
   gap: 8px;
   color: var(--text-muted);
   font-size: 13px;
+  /* Reset browser <button> defaults — done/skipped steps render as buttons
+     (clickable to review) and would otherwise pick up the OS button
+     chrome (white background, system border, etc). */
+  background: transparent;
+  border: none;
+  padding: 0;
+  font-family: inherit;
 }
 
 .model-onboarding-step-indicator.active {