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 {

packages/dashboard/app/components/SettingsModal.css

@@ -690,7 +690,7 @@
   border-left: 3px solid var(--text-muted);
 }
 .auth-provider-group {
-  margin-bottom: 16px;
+  margin-bottom: 10px;
 }
 .auth-group-label {
   font-size: 11px;
@@ -698,14 +698,14 @@
   text-transform: uppercase;
   letter-spacing: 0.5px;
   color: var(--text-muted);
-  margin-bottom: 8px;
+  margin-bottom: 4px;
   padding: 0 4px;
 }
 .auth-provider-card {
   background: var(--surface);
   border: 1px solid var(--border);
   border-radius: var(--radius-md);
-  margin-bottom: 8px;
+  margin-bottom: 4px;
   overflow: hidden;
   transition: border-color var(--transition-fast), box-shadow var(--transition-fast);
 }
@@ -723,7 +723,7 @@
   display: flex;
   align-items: center;
   justify-content: space-between;
-  padding: 12px 16px;
+  padding: 8px 12px;
   gap: 12px;
 }
 .auth-provider-info {