🤖 refactor: consolidate Storybook to full-app only approach (#823)

## Summary

Consolidate all Storybook stories to full-app only - no more isolated
component stories. Every visual state is now tested in context, showing
how changes will actually appear to users.

## Changes

**Deleted (22 files, ~5,800 lines)**
- All individual component stories in `src/browser/components/`
- Original `src/browser/App.stories.tsx`

**Created (2 files, ~1,800 lines)**
- `src/browser/stories/mockFactory.ts` - Composable mock infrastructure
- `src/browser/stories/App.stories.tsx` - 17 full-app stories

## New Story Organization

| Area | Stories |
|------|---------|
| Empty/Welcome | WelcomeScreen |
| Sidebar & Navigation | SingleProject, MultipleProjects, ManyWorkspaces
|
| Git Status | GitStatusVariations (clean, ahead, behind, dirty,
diverged, SSH) |
| Chat Messages | ChatConversation, ChatWithReasoning, ChatWithTerminal,
ChatWithAgentStatus |
| Streaming | StreamingState (pending tool calls) |
| Markdown | MarkdownTables, CodeBlocks |
| Media | MessageWithImages |
| Errors | StreamError, HiddenHistory |
| Diffs | LargeDiff |
| Comprehensive | ComprehensiveDemo (exercises everything) |

## Benefits

1. **Context is king** - Every visual state shown in full app
2. **DRY mock infrastructure** - Composable factories eliminate
duplication
3. **Maintainable** - Adding stories = combining existing building
blocks
4. **Realistic** - Stories exercise real app behavior
5. **~70% less code** - 1,800 lines vs ~5,800 previously

---
_Generated with `mux`_

Author: ammar-agent

bun.lock

@@ -101,6 +101,7 @@
     "@storybook/addon-docs": "^10.0.0",
     "@storybook/addon-links": "^10.0.0",
     "@storybook/react-vite": "^10.0.0",
+    "@storybook/test": "^8.6.14",
     "@storybook/test-runner": "^0.24.0",
     "@tailwindcss/vite": "^4.1.15",
     "@testing-library/react": "^16.3.0",