feat: add /backend command parsing

Add support for /backend command to show or switch local backend providers. Follows the same pattern as /model command with optional argument support.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/frontends/tui/commands.ts

@@ -5,6 +5,7 @@ export type Command =
   | { type: 'status' }
   | { type: 'fullscreen' }
   | { type: 'model'; name?: string }
+  | { type: 'backend'; provider?: string }
   | { type: 'transfer'; target: string }
   | { type: 'message'; content: string };
 
@@ -46,6 +47,15 @@ export function parseCommand(input: string): Command | null {
     return { type: 'model', name };
   }
 
+  // Backend (with optional argument)
+  if (trimmed === '/backend') {
+    return { type: 'backend' };
+  }
+  if (trimmed.startsWith('/backend ')) {
+    const provider = trimmed.slice('/backend '.length).trim();
+    return { type: 'backend', provider };
+  }
+
   // Transfer
   if (trimmed.startsWith('/transfer ')) {
     const target = trimmed.slice('/transfer '.length).trim();
@@ -61,6 +71,7 @@ export function getHelpText(): string {
 Commands:
   /help, /?          Show this help
   /model [name]      Show or switch model (local, default, fast, complex)
+  /backend [provider]    Show or switch local backend (ollama, llamacpp)
   /reset, /clear     Clear conversation history
   /status            Show session info and token usage
   /fullscreen, /fs   Switch to fullscreen mode