feat: East Africa + Southern Africa pills with regional sub-sections

- East Africa pill: merges Uganda + east-africa tags; Uganda pinned first via sub-sections
- Southern Africa pill: covers SA, Namibia, Botswana, Zimbabwe, Zambia, Mozambique; SA/Namibia/Botswana prioritized
- StoryStore.setPill now resets lastSyncedAt to bypass 30s rate-limit on filter switch
- sectionSupplement background fetch populates sparse regional sections in All digest
- SignalFeedView digestContent: flat hero+scroll path for pills without subSections (was showing max 5)
- Theme: removed standalone Uganda pill, wired new sub-section layouts

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Jarvis/Views/Onboarding/OnboardingView.swift

@@ -11,7 +11,7 @@ struct OnboardingView: View {
 
     var body: some View {
         ZStack {
-            Color.black.ignoresSafeArea()
+            Palette.background.ignoresSafeArea()
 
             VStack(alignment: .leading, spacing: 0) {
                 Spacer().frame(height: 48)