fix: don't crash on startup when stdout can't encode the banner (#25482)

The startup banner uses Unicode box-drawing characters. On a stdout that
can't encode them (Windows cp1252, or redirected/headless/pythonw output)
print() raises UnicodeEncodeError and aborts startup. This blocks running
open-webui serve headless on Windows.

Guard the banner print and fall back to a plain ASCII line so startup
always proceeds regardless of the console encoding.

Fixes #24965

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/open_webui/main.py

@@ -608,7 +608,7 @@ class SPAStaticFiles(StaticFiles):
 
 
 if LOG_FORMAT != 'json':
-    print(rf"""
+    banner = rf"""
  ██████╗ ██████╗ ███████╗███╗   ██╗    ██╗    ██╗███████╗██████╗ ██╗   ██╗██╗
 ██╔═══██╗██╔══██╗██╔════╝████╗  ██║    ██║    ██║██╔════╝██╔══██╗██║   ██║██║
 ██║   ██║██████╔╝█████╗  ██╔██╗ ██║    ██║ █╗ ██║█████╗  ██████╔╝██║   ██║██║
@@ -620,7 +620,12 @@ if LOG_FORMAT != 'json':
 v{VERSION} - building the best AI user interface.
 {f'Commit: {WEBUI_BUILD_HASH}' if WEBUI_BUILD_HASH != 'dev-build' else ''}
 https://github.com/open-webui/open-webui
-""")
+"""
+    try:
+        print(banner)
+    except UnicodeEncodeError:
+        # Stdout can't encode the box-drawing banner (Windows cp1252, redirected/headless stdout); fall back to ASCII.
+        print(f'Open WebUI v{VERSION} - building the best AI user interface.\nhttps://github.com/open-webui/open-webui')
 
 
 @asynccontextmanager