fix: Windows path handling, light theme default, file viewer in editor, first-user admin logic

Config:
- Fixed SQLite path parsing on Windows (handle drive letters without prepending /)
- Changed default theme from dark to light
- Updated OAuth test credentials (gitea_oauth_client_id/secret)

Auth:
- First real user (excluding default admin with no password) becomes admin
- Changed user count query to exclude users without password_hash

File viewer:
- Removed separate _render_file_viewer() —

app/routers/auth.py

@@ -153,9 +153,11 @@ async def register(request: Request):
         if existing:
             from fastapi.responses import JSONResponse
             return JSONResponse({"error": "Account already exists"}, status_code=409)
-        # First user is admin
-        user_count = conn.execute("SELECT COUNT(*) FROM users").fetchone()[0]
-        is_admin = 1 if user_count == 0 else 0
+        # First real user (excluding default admin with no password) is admin
+        real_user_count = conn.execute(
+            "SELECT COUNT(*) FROM users WHERE password_hash IS NOT NULL AND password_hash != ''"
+        ).fetchone()[0]
+        is_admin = 1 if real_user_count == 0 else 0
         conn.execute(
             "INSERT INTO users (login, full_name, email, password_hash, is_admin) VALUES (?, ?, ?, ?, ?)",
             (email, name, email, hash_password(password), is_admin),