perf(core): Route code-splitting, lazy visualizers, Cmd+K palette & chat refactor

[arvarik]

Summary of PR 1

1. Route-Level Code Splitting:

   • Converted all 9 top-level routes in src/App.jsx to React.lazy() wrapped with Suspense and a spinner fallback.
   • Initial production bundle dropped from 1.91 MB to 161.16 kB (a 91.6% reduction).
   • Configured custom chunking for heavy vendor modules (@xyflow, katex, react-syntax-highlighter, lucide-react, react-router, react).

2. Lazy-Loaded Visualizers:

   • Converted Mermaid diagrams in src/components/shared/MermaidRenderer.jsx to load the heavy mermaid package on demand only when a diagram actually appears.

3. Universal Command Palette (Cmd+K / Ctrl+K):

   • Bound Cmd+K / Ctrl+K across all pages to trigger the global search / quick switcher modal.
   • Enhanced SearchDialog with direct links to all core pages with keyboard shortcut badges.
   • Preserved Cmd+J for Chat toggle and added hints in shortcuts modal.

4. Component Decomposition:

   • Extracted PersonaPicker and SessionPicker out of the 1,220-line LearningChat.jsx into modular, testable sub-components with dedicated personas.js constants.

Verification

• npm run lint passed (0 errors, 0 warnings).
• npm test passed (18 test suites, 248 passed).
• npm run build passed successfully.