feat(navigator): unified omni-search command palette and full source code viewer

[spelech]

Summary

Enhances the 3-Pane Codebase Navigator with universal Omni-Search across AST symbols, files, routes, and raw source code, paired with a full source code viewer with target line highlighting and caller/callee impact drawer.

Features

1. Unified Omni-Search (GET /admin/api/navigator/omni-search & NavigatorOmniSearch):

   • Real-time search across AST symbols, files, API routes, and code chunks with normalized confidence scoring (99% AST exact, 94% AST prefix, 92% filename, 88% code substring).
   • Floating command palette overlay (position: absolute; z-index: 50) with keyboard navigation (ArrowUp/ArrowDown/Enter/Escape), match kind chips, and zero header layout shift.
   • Jump directly to target file and symbol declaration line.

2. Full Source Code Viewer (NavigatorCodeViewer):

   • 1-based line numbered code viewer for code files in Pane 3 (preserving NavigatorDocReader for markdown/docs).
   • Smooth auto-scrolling to target line range (targetStartLine / targetEndLine) with amber highlight styling.
   • Collapsible docked impact drawer displaying incoming caller and outgoing callee counts with instant click-to-jump.
   • Copy code snippet and deep link copy buttons.
   • Complete mobile word-wrapping safeguarding against overflow on narrow mobile viewports.

3. Documentation & Tests:

   • Updated docs/guide/user-guide/navigator.md documenting Omni-Search and Code Viewer.
   • Backend pytest unit tests in tests/backend/test_navigator_router.py.
   • Vitest component tests in NavigatorOmniSearch.test.tsx and NavigatorCodeViewer.test.tsx (300/300 passing).
   • Playwright E2E layout audit in e2e/navigator.spec.ts (36/36 passing).