feat(shell): restore AI assistant panel

Re-add the AI assistant panel that was removed in previous refactoring.
The panel includes:
- Message history display
- Input field for AI queries
- Loading state indicator

Also restored the associated CSS styles and i18n translations.

Assisted-by: MiniMax-M2.7 via Crush <crush@charm.land>

web/src/i18n/fr.js

@@ -107,6 +107,9 @@ const fr = {
     systemTerminals: 'Terminaux syst\u00e8me',
     switchTerminal: 'Changer de terminal',
     localShell: 'Shell local',
+    aiAssistant: 'Assistant IA',
+    aiWelcome: 'Bonjour ! Je peux vous aider avec les commandes du terminal. Demandez-moi n\'importe quoi !',
+    askAi: 'Interroger l\'assistant IA...',
   },
 
   config: {