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-container-status.demo.ts b/webui/src/web-components/demo/sketch-container-status.demo.ts
index da0e435..0b49f21 100644
--- a/webui/src/web-components/demo/sketch-container-status.demo.ts
+++ b/webui/src/web-components/demo/sketch-container-status.demo.ts
@@ -45,7 +45,8 @@
 
     const lightLabel = document.createElement("h4");
     lightLabel.textContent = "Light Usage";
-    lightLabel.style.cssText = "margin: 20px 0 10px 0; color: #24292f;";
+    lightLabel.style.cssText =
+      "margin: 20px 0 10px 0; color: var(--demo-label-color);";
 
     // Heavy usage status
     const heavyStatus = document.createElement(
@@ -59,7 +60,8 @@
     };
     const heavyLabel = document.createElement("h4");
     heavyLabel.textContent = "Heavy Usage";
-    heavyLabel.style.cssText = "margin: 20px 0 10px 0; color: #24292f;";
+    heavyLabel.style.cssText =
+      "margin: 20px 0 10px 0; color: var(--demo-label-color);";
 
     // Control buttons for interaction
     const controlsDiv = document.createElement("div");