style(ui): lift section-header alpha 0.5->0.6 to match muted-label tier

Section headers and the expandable chevron were muted to onSurfaceVariant
@ 0.5 — a notch below the app's own secondary-text floor (theme.dart uses
0.6-0.7 for hint/leading/trailing labels) and borderline for 12px
uppercase against WCAG AA. 0.6 keeps the muted look while landing on the
existing leadingAndTrailingTextStyle tier. TabBars already use M3 default
primary-on-selected (consistent with the new bottom nav); nav inactive
icons stay at 0.5 (24px glyphs, intentional).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/widgets/expandable_section_header.dart

@@ -53,7 +53,7 @@ class ExpandableSectionHeader extends StatelessWidget {
                       fontWeight: FontWeight.w500,
                       letterSpacing: 0.8,
                       color: context.colorScheme.onSurfaceVariant.withValues(
-                        alpha: 0.5,
+                        alpha: 0.6,
                       ),
                     ),
                   ),
@@ -64,7 +64,7 @@ class ExpandableSectionHeader extends StatelessWidget {
                   child: Icon(
                     Icons.keyboard_arrow_down,
                     color: context.colorScheme.onSurfaceVariant.withValues(
-                      alpha: 0.5,
+                      alpha: 0.6,
                     ),
                     size: 20,
                   ),

lib/widgets/section_header.dart

@@ -40,7 +40,7 @@ class SectionHeader extends StatelessWidget {
               fontWeight: FontWeight.w500,
               letterSpacing: 0.8,
               color: context.colorScheme.onSurfaceVariant.withValues(
-                alpha: 0.5,
+                alpha: 0.6,
               ),
             ),
           ),