feat(FN-2964): merge fusion/fn-2964

Commits merged:
- test(FN-2964): complete Step 2 — cover custom providers disclosure behavior
- feat(FN-2964): complete Step 1 — embed custom providers disclosure in onboarding
- fix(FN-2963): reuse shared onboarding disclosure for custom providers
- fix(FN-2963): address disclosure focus ring and mobile touch target
- fix(FN-2963): align custom providers disclosure with shared pattern
- feat(FN-2963): apply inline edit UX and tokenized custom provider styles
- fix(FN-2963): align custom providers UI with auth patterns
- fix(FN-2963): complete Step 5 — resolve lint gate in providers section
- test(FN-2963): complete Step 4 — add custom providers section tests
- feat(FN-2963): complete Step 3 — integrate custom providers section
- feat(FN-2963): complete Step 2 — add custom providers section styles
- feat(FN-2963): complete Step 1 — create custom providers section component
- feat(FN-2961): complete Step 6 — document custom provider settings
- test(FN-2961): complete Step 4 — cover custom provider routes
- feat(FN-2961): complete Step 3 — add custom provider API client
- feat(FN-2961): complete Step 2 — add custom provider CRUD routes
- feat(FN-2961): complete Step 1 — add custom provider core types

Files changed:
.../app/components/CustomProvidersSection.css      | 138 +++++++
 .../app/components/CustomProvidersSection.tsx      | 425 +++++++++++++++++++++
 .../app/components/ModelOnboardingModal.css        |  60 ---
 .../app/components/OnboardingDisclosure.css        |  67 ++++
 .../app/components/OnboardingDisclosure.tsx        |  38 ++
 .../__tests__/CustomProvidersSection.test.tsx      | 237 ++++++++++++
 .../__tests__/ModelOnboardingModal.test.tsx        |  67 +++-
 7 files changed, 967 insertions(+), 65 deletions(-)

Fusion-Task-Id: FN-2964

packages/dashboard/app/components/ModelOnboardingModal.css

@@ -218,66 +218,6 @@
 }
 
 
-/* === OnboardingDisclosure === */
-.onboarding-disclosure {
-  display: flex;
-  flex-direction: column;
-  margin-top: var(--space-sm);
-}
-
-.onboarding-disclosure-trigger {
-  display: flex;
-  align-items: center;
-  gap: var(--space-xs);
-  background: none;
-  border: none;
-  color: var(--text-muted);
-  font-size: 12px;
-  cursor: pointer;
-  padding: var(--space-xs) 0;
-  transition: color var(--transition-fast);
-  font-family: inherit;
-}
-
-.onboarding-disclosure-trigger:hover {
-  color: var(--text);
-}
-
-.onboarding-disclosure-trigger:focus-visible {
-  outline: var(--focus-ring-strong);
-  border-radius: var(--radius-sm);
-}
-
-.onboarding-disclosure-chevron {
-  width: 14px;
-  height: 14px;
-  transition: transform var(--transition-fast);
-  flex-shrink: 0;
-}
-
-.onboarding-disclosure-trigger[aria-expanded="true"] .onboarding-disclosure-chevron {
-  transform: rotate(90deg);
-}
-
-.onboarding-disclosure-content {
-  padding: var(--space-sm) 0 var(--space-sm) 22px;
-  color: var(--text-muted);
-  font-size: 12px;
-  line-height: 1.5;
-  animation: onboarding-disclosure-enter 150ms ease-out;
-}
-
-@keyframes onboarding-disclosure-enter {
-  from {
-    opacity: 0;
-    transform: translateY(-4px);
-  }
-  to {
-    opacity: 1;
-    transform: translateY(0);
-  }
-}
-
 .model-onboarding-loading {
   display: flex;
   align-items: center;