🤖 refactor: split Storybook into focused full-app story files

Consolidate all component stories into full-app stories, then split the
mega-file into focused story files organized by functional area.

src/browser/stories/
├── mockFactory.ts      (542 lines) - Composable mock builders
├── storyHelpers.ts     (189 lines) - Reusable setup patterns
├── meta.tsx            (47 lines)  - Shared Storybook config
├── App.welcome.stories.tsx  - Empty/welcome states
├── App.sidebar.stories.tsx  - Project navigation, git status
├── App.chat.stories.tsx     - Conversations, tools, reasoning
├── App.markdown.stories.tsx - Tables, code blocks
├── App.media.stories.tsx    - Image attachments
├── App.errors.stories.tsx   - Stream errors, hidden history
└── App.demo.stories.tsx     - Comprehensive feature demo

- setupSimpleChatStory() - One workspace with static messages
- setupStreamingChatStory() - Active streaming state
- setupCustomChatStory() - Custom chat handler for edge cases
- selectWorkspace() - Set localStorage for workspace selection

Each story file is now ~100-200 lines, focused on a single area.

_Generated with `mux`_

Author: ammar-agent