feat(frontend): Scholarchat-inspired UI refactor with top nav, sidebar, and message actions

- Add TopNav component with AI Chat, Study Guides, Progress tabs
- Refactor sidebar to include brand, new chat, subjects, chat history, settings
- Add message action buttons (Helpful, Note, Deeper) on assistant responses
- Enhance ChatInput with photo, mic attachment icons and AI disclaimer
- Improve welcome state with 2x2 suggested prompt grid
- Update global layout for proper sidebar offset (fix nav overlap)
- Remove obsolete Header component
- All styles: strict BEM naming, zero inline CSS, Paper Look theme

.gitignore

@@ -0,0 +1,24 @@
+# Logs
+logs
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+pnpm-debug.log*
+lerna-debug.log*
+
+node_modules
+dist
+dist-ssr
+*.local
+
+# Editor directories and files
+.vscode/*
+!.vscode/extensions.json
+.idea
+.DS_Store
+*.suo
+*.ntvs*
+*.njsproj
+*.sln
+*.sw?