fix(avatars): use proper getThumbnailUrl API to eliminate 404 errors

Root cause: Widgets were using getUserAvatar() which returns bare
filenames like 'user-default.png'. These were being used directly as
image src URLs, causing 404 errors like /user-default.png.

Solution: Use getAvatarUrl() dependency which calls getThumbnailUrl()
to convert filenames to proper URLs like /thumbnail?type=persona&file=...

Changes:
- userInfoWidget.js: Use getAvatarUrl('persona', rawAvatar) instead of
  raw avatar validation
- sillytavern.js: Simplify updatePersonaAvatar() to trust
  getSafeThumbnailUrl() which already calls getThumbnailUrl()

This eliminates 404 errors on initial render and when switching tabs.

src/systems/dashboard/widgets/userInfoWidget.js

@@ -26,6 +26,7 @@ export function registerUserInfoWidget(registry, dependencies) {
     const {
         getContext,
         getUserAvatar,
+        getAvatarUrl,
         getFallbackAvatar,
         getExtensionSettings,
         onStatsChange
@@ -50,7 +51,16 @@ export function registerUserInfoWidget(registry, dependencies) {
             const settings = getExtensionSettings();
             const context = getContext();
             const userName = context.name1;
-            const userPortrait = getUserAvatar() || getFallbackAvatar();
+
+            // Get user avatar - use getAvatarUrl to convert filename to proper thumbnail URL
+            let userPortrait = getFallbackAvatar();
+            const rawAvatar = getUserAvatar();
+
+            // Convert raw avatar filename to proper thumbnail URL
+            // getAvatarUrl calls getThumbnailUrl which generates URLs like /thumbnail?type=persona&file=...
+            if (rawAvatar) {
+                userPortrait = getAvatarUrl('persona', rawAvatar);
+            }
 
             // Merge default config
             const finalConfig = {