fix: MCP server compatibility and web UI path resolution

Fixes #371, #369

**Issue #371: MCP server fails when Bun not in PATH**
- Changed MCP server shebang from `#!/usr/bin/env bun` to `#!/usr/bin/env node`
- MCP server now works regardless of whether Bun is in PATH
- Worker service correctly uses getBunPath() to find Bun in common install locations

**Issue #369: Web UI returns ENOENT error**
- Fixed hardcoded 'plugin/' path in ViewerRoutes
- Now checks both cache structure (ui/viewer.html) and marketplace structure (plugin/ui/viewer.html)
- Web UI now works from both ~/.claude/plugins/cache and ~/.claude/plugins/marketplaces

**Technical Details:**
- Updated build-hooks.js to use Node shebang for MCP server (line 169)
- Enhanced ViewerRoutes.handleViewerUI() to try multiple path patterns
- Added existsSync check to find viewer.html in either location

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

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