fix(ai): persist API keys to disk after set/remove operations

API keys set via /ai set token were only stored in memory and lost
on client restart. prefs_ai_set_token() and prefs_ai_remove_token()
mutated the in-memory prefs but never called _save_prefs(), unlike
other preference setters in the same file.

Both functions now call _save_prefs() after mutating the keyfile,
ensuring API keys persist across client restarts as advertised.

src/config/preferences.c

@@ -1722,6 +1722,7 @@ prefs_ai_set_token(const char* const provider, const char* const token)
 
     if (token) {
         g_key_file_set_string(prefs, PREF_GROUP_AI, provider, token);
+        _save_prefs();
         return TRUE;
     } else {
         return prefs_ai_remove_token(provider);
@@ -1742,6 +1743,7 @@ prefs_ai_remove_token(const char* const provider)
     }
 
     g_key_file_remove_key(prefs, PREF_GROUP_AI, provider, NULL);
+    _save_prefs();
     return TRUE;
 }