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

.oxlintrc.json

@@ -0,0 +1,8 @@
+{
+  "$schema": "./node_modules/oxlint/configuration_schema.json",
+  "plugins": ["react", "oxc"],
+  "rules": {
+    "react/rules-of-hooks": "error",
+    "react/only-export-components": ["warn", { "allowConstantExport": true }]
+  }
+}