feat(ui): redesign chat, reports, settings, and biomarker screens

Applies the Stitch mockup design system across all remaining app screens,
replacing the default Material theme colors with the Koshika palette via
AppColors. Changes are visual-only; all logic, state, and navigation remain
untouched.

chat_screen (all 6 model lifecycle states):
- State views (not-downloaded, downloading, ready, loading, error) now use
  circular icon containers with AppColors.primaryContainer tinted backgrounds
  instead of bare icons, giving each state a distinct, branded look.
- Download progress bar uses AppColors.primary as the fill color.
- Search mode indicator bar switches between a tinted primary background
  (semantic search active) and a neutral surface color (keyword fallback).
- Input field fill changed to AppColors.surfaceContainerLow; send button
  explicitly styled with AppColors.primary when active.
- Suggestion chips in the empty chat state use AppColors.surfaceContainerLow
  fill + AppColors.outlineVariant border for a softer appearance.

reports_screen:
- Empty state uses the same circular icon container pattern (primary tint).
- FAB explicitly styled AppColors.primary / white to override theme default.
- Swipe-to-delete background uses AppColors.error with a rounded decoration
  instead of a flat Colors.red fill.
- Report list cards use white fill, AppColors.primary icons, and
  AppColors.onSurface / onSurfaceVariant text for consistent typography.

settings_screen:
- Section header icon and label color updated to AppColors.primary.
- Model status badge colors replaced: loaded → AppColors.statusActive
  (dark teal), ready → AppColors.statusReady (blue), downloading/loading →
  AppColors.statusBusy (amber), error → AppColors.error. Applies to both
  the Chat model tile and the Embedding model tile.

biomarker_detail_screen:
- Latest value displayed in AppColors.primary; unit label in onSurfaceVariant.
- History list row tint colors aligned to the palette: normal rows use
  AppColors.statusActive at low alpha, borderline/low rows use
  AppColors.statusBusy, high/critical rows use AppColors.error.

Author: priyavratuniyal

lib/screens/biomarker_detail_screen.dart

@@ -6,6 +6,8 @@ import '../widgets/flag_badge.dart';
 import '../widgets/biomarker_trend_chart.dart';
 import '../widgets/reference_range_gauge.dart';
 
+import '../theme/app_colors.dart';
+
 class BiomarkerDetailScreen extends StatefulWidget {
   final String biomarkerKey;
 
@@ -168,15 +170,15 @@ class _BiomarkerDetailScreenState extends State<BiomarkerDetailScreen> {
                             latestResult.formattedValue,
                             style: theme.textTheme.displaySmall?.copyWith(
                               fontWeight: FontWeight.bold,
-                              color: theme.colorScheme.primary,
+                              color: AppColors.primary,
                             ),
                           ),
                           const SizedBox(width: 8),
                           if (latestResult.unit != null)
                             Text(
                               latestResult.unit!,
                               style: theme.textTheme.titleMedium?.copyWith(
-                                color: theme.colorScheme.onSurfaceVariant,
+                                color: AppColors.onSurfaceVariant,
                               ),
                             ),
                         ],
@@ -298,15 +300,15 @@ class _BiomarkerDetailScreenState extends State<BiomarkerDetailScreen> {
   Color _flagRowColor(BiomarkerFlag flag) {
     switch (flag) {
       case BiomarkerFlag.normal:
-        return Colors.green.withValues(alpha: 0.06);
+        return AppColors.statusActive.withValues(alpha: 0.06);
       case BiomarkerFlag.borderline:
-        return Colors.amber.withValues(alpha: 0.10);
+        return AppColors.statusBusy.withValues(alpha: 0.08);
       case BiomarkerFlag.low:
-        return Colors.orange.withValues(alpha: 0.08);
+        return AppColors.statusBusy.withValues(alpha: 0.08);
       case BiomarkerFlag.high:
-        return Colors.red.withValues(alpha: 0.08);
+        return AppColors.error.withValues(alpha: 0.07);
       case BiomarkerFlag.critical:
-        return Colors.red.withValues(alpha: 0.14);
+        return AppColors.error.withValues(alpha: 0.12);
       case BiomarkerFlag.unknown:
         return Colors.transparent;
     }