webui: add dark mode support to demo server

Core Component Dark Mode Support:
- sketch-timeline.ts: Welcome box, loading indicators, thinking bubbles, navigation
- sketch-tool-card-base.ts: Status icons, elapsed time, hover states, details panel
- All 14 sketch-tool-card-* components: Consistent dark styling for tool results

Demo System Infrastructure:
- Enhanced demo runner (demo.html) with complete dark theme CSS variables
- Added sketch-theme-toggle integration in sidebar for easy theme switching
- Extended demo-fixtures utilities with semantic color system (8 new CSS variables)
- Comprehensive color mappings: backgrounds, text, borders, controls, buttons

Demo File Compatibility (13 files updated):
- Fixed 60+ instances of hardcoded colors across all demo components
- Replaced light-mode-only colors (#24292f, #f6f8fa, etc.) with CSS variables
- Updated text colors, backgrounds, borders for proper contrast in both themes
- Maintained visual hierarchy while ensuring accessibility

Technical Implementation:
- CSS custom properties system with automatic :root/.dark theme switching
- GitHub-inspired dark color palette for professional appearance
- Smooth 0.2s transitions for seamless theme changes
- Semantic variable naming for maintainability and consistency

Key Features Added:
- Theme toggle accessible from any demo (no need to navigate to Theme Toggle demo)
- Complete visual consistency between light and dark modes
- Proper contrast ratios throughout for accessibility
- Tool card demos showcase dark mode styling with realistic content

Components Updated:
Timeline: Welcome messages, loading states, thinking indicators, jump button
Tool Cards: Status icons, input/output display, hover states, detailed views
Demos: Labels, backgrounds, instruction panels, control elements, text content

The demo system now provides a complete, professional dark mode experience
that matches modern development tool standards with excellent usability
and visual consistency across all components and demonstrations.

Co-Authored-By: sketch <hello@sketch.dev>
Change-ID: s97589e2fe2fdeeb3k
diff --git a/webui/src/web-components/demo/sketch-timeline-viewport.demo.ts b/webui/src/web-components/demo/sketch-timeline-viewport.demo.ts
index cc4b519..e8d4a8f 100644
--- a/webui/src/web-components/demo/sketch-timeline-viewport.demo.ts
+++ b/webui/src/web-components/demo/sketch-timeline-viewport.demo.ts
@@ -21,8 +21,8 @@
     }
     .demo-header {
       padding: 20px;
-      border-bottom: 1px solid #eee;
-      background: #f8f9fa;
+      border-bottom: 1px solid var(--demo-border);
+      background: var(--demo-fixture-section-bg);
       border-radius: 8px 8px 0 0;
     }
     .demo-timeline {
@@ -31,8 +31,8 @@
     }
     .controls {
       padding: 10px 20px;
-      border-top: 1px solid #eee;
-      background: #f8f9fa;
+      border-top: 1px solid var(--demo-border);
+      background: var(--demo-fixture-section-bg);
       display: flex;
       gap: 10px;
       align-items: center;
@@ -50,7 +50,7 @@
     }
     .info {
       font-size: 12px;
-      color: #666;
+      color: var(--demo-secondary-text);
       margin-left: auto;
     }
   `,