feat: add Storybook for webview components to enable isolated UI development

[charannyk06]

Priority: P3 — Low

Problem

The webview has ~50 React components but no way to view or develop them in isolation. Developers must:

1. Build the full extension
2. Open a VS Code window with the extension loaded
3. Trigger the specific UI state needed

This is extremely slow for UI development. A FindingCard with complex states (expanded, with patch, with validation errors, loading) requires specific data that's hard to reproduce manually.

Solution

1. Install Storybook

npx storybook@latest init --type vite

2. Add stories for key components

Priority components for stories:

• FindingCard — all severity levels, with/without patch, applied state, error state
• ChatMessage — user/assistant, with reasoning blocks, with tool calls
• EvidencePanel — with code snippet, file path, line numbers
• DiffViewer — show before/after patch
• AuditTimeline — various event types
• PatchPreview — valid patch, patch with errors

3. Mock data

Create src/webview/stories/fixtures.ts with realistic mock findings, chat messages, and review runs.

4. Add to scripts

"storybook": "storybook dev -p 6006",
"build:storybook": "storybook build"

Acceptance Criteria

• Storybook installed and running (npm run storybook)
• Stories for the 6 key components above
• Mock data fixtures file created
• Interactive controls for component props
• Storybook build added to CI (optional, for visual regression)