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/Shared/SignalStripe.swift

@@ -29,5 +29,5 @@ struct SignalStripe: View {
         }
     }
     .padding()
-    .background(Color.black)
+    .background(Palette.background)
 }