webui: convert SketchTimeline to use TailwindElement and Tailwind CSS classes

Convert SketchTimeline component from Lit CSS-in-JS styles to TailwindElement
inheritance with Tailwind utility classes, replacing shadow DOM styling with
global Tailwind CSS classes.

Problems Solved:

CSS Inconsistency:
- SketchTimeline used shadow DOM with CSS-in-JS styles while other components use TailwindElement
- Component styling was isolated from global design system
- Difficult to maintain consistent visual appearance across components
- No access to global Tailwind utility classes within shadow DOM

Test Brittleness:
- Tests relied on CSS class selectors that were implementation details
- Complex CSS class selectors made tests fragile to styling changes
- No standardized approach for testing UI elements across components

Missing Demo Infrastructure:
- SketchTimeline had no TypeScript demo module for component development
- Component not included in demo runner system for iterative development
- Only had static HTML demo without interactive controls

Solution Implementation:

TailwindElement Conversion:
- Changed inheritance from LitElement to SketchTailwindElement to disable shadow DOM
- Replaced all CSS-in-JS styles with equivalent Tailwind utility classes
- Added custom CSS for complex animations (thinking dots, loading spinner) that can't be easily replicated with Tailwind
- Maintained all existing visual styling and behavior while using Tailwind classes

CSS Class Mapping:
- .timeline-container → w-full relative max-w-full mx-auto px-[15px] box-border overflow-x-hidden flex-1 min-h-[100px]
- .welcome-box → my-8 mx-auto max-w-[90%] w-[90%] p-8 border-2 border-gray-300 rounded-lg shadow-sm bg-white text-center
- .thinking-indicator → pl-[85px] mt-1.5 mb-4 flex
- .loading-indicator → flex items-center justify-center p-5 text-gray-600 text-sm gap-2.5 opacity-100
- Added print: utility variants for print styling support

Test Infrastructure Updates:
- Replaced CSS class selectors with data-testid attributes for reliable element targeting
- Updated all test selectors to use [data-testid='element-name'] pattern
- Added test IDs to welcome-box, timeline-container, thinking-indicator, loading-indicator, thinking-bubble, thinking-dots, and thinking-dot elements
- Maintained all existing test functionality while improving test reliability

Demo Module Creation:
- Created sketch-timeline.demo.ts with comprehensive interactive demo
- Implemented basic timeline, loading states, thinking states, and interactive controls
- Added mock message generation with various message types and tool calls
- Included controls for adding messages, toggling thinking state, compact padding, and reset functionality
- Added SketchTimeline to knownComponents list in demo-runner.ts

Custom Styling Architecture:
- Added addCustomStyles() method to inject necessary CSS that can't be replicated with Tailwind
- Created thinking-pulse keyframe animation for thinking dots
- Added loading-spin animation for spinner elements
- Implemented compact-padding responsive styling
- Used document.head.appendChild for global style injection with duplicate prevention

Implementation Details:

Component Structure:
- Maintained all existing properties, methods, and component lifecycle
- Preserved scroll handling, viewport management, and loading operations
- Added data-testid attributes without affecting visual presentation
- Kept all existing functionality while changing only the styling approach

Styling Consistency:
- All colors, spacing, borders, and animations maintained visual parity
- Print styles converted to Tailwind print: variants
- Hover and active states preserved with Tailwind state variants
- Responsive design maintained with existing breakpoint behavior

Test Reliability:
- Test selectors now target semantic element roles rather than implementation details
- More robust element identification reduces test flakiness
- Consistent testing pattern across all timeline-related components
- Better separation between styling and testing concerns

Demo Development:
- Interactive demo supports real-time component behavior testing
- Mock data factory functions for consistent test data generation
- Multiple demo scenarios covering empty state, populated timeline, and various loading states
- Control buttons for testing user interactions and state changes

Files Modified:
- sketch/webui/src/web-components/sketch-timeline.ts: TailwindElement inheritance and Tailwind class conversion
- sketch/webui/src/web-components/sketch-timeline.test.ts: Updated test selectors to use data-testid attributes
- sketch/webui/src/web-components/demo/sketch-timeline.demo.ts: New interactive demo module
- sketch/webui/src/web-components/demo/demo-framework/demo-runner.ts: Added sketch-timeline to knownComponents

The conversion maintains complete visual and functional parity while enabling
consistent styling across the component library and improving test reliability
through semantic element targeting.

Co-Authored-By: sketch <hello@sketch.dev>
Change-ID: s0621383cac6304dek
diff --git a/webui/package.json b/webui/package.json
index a6d0fe5..6454fb8 100644
--- a/webui/package.json
+++ b/webui/package.json
@@ -23,7 +23,7 @@
     "build": "go run ../cmd/go2ts -o src/types.ts && tsc",
     "watch": "tsc --watch",
     "tailwind": "tailwindcss -i ./src/global.css -o ./dist/tailwind.css",
-    "test": "tsc && npm run test:playwright",
+    "test": "tsc && tailwindcss -i ./src/global.css -o ./dist/tailwind.css && npm run test:playwright",
     "test:playwright": "playwright test -c playwright-ct.config.ts"
   },
   "dependencies": {