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/SourceChips.swift

@@ -26,11 +26,11 @@ struct SourceChips: View {
 
     private func chip(_ text: String, muted: Bool = false) -> some View {
         Text(text)
-            .font(.system(size: 12, weight: .bold))
-            .foregroundStyle(muted ? Color(hex: "777777") : Color(hex: "CCCCCC"))
-            .padding(.horizontal, 9)
-            .padding(.vertical, 5)
-            .background(Palette.surface2)
+            .font(.system(size: 11, weight: .semibold))
+            .foregroundStyle(muted ? Palette.chipMutedText : Palette.chipText)
+            .padding(.horizontal, 8)
+            .padding(.vertical, 3)
+            .background(Palette.chipFill)
             .clipShape(Capsule())
     }
 }
@@ -38,5 +38,5 @@ struct SourceChips: View {
 #Preview {
     SourceChips(names: ["Daily Monitor", "NilePost", "The EastAfrican"], total: 7)
         .padding()
-        .background(Color.black)
+        .background(Palette.background)
 }