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/config.py

@@ -13,8 +13,8 @@ class Settings(BaseSettings):
     # Gitea
     gitea_url: str = "https://git.dracodev.net"
     gitea_token: str = "change-me"
-    gitea_oauth_client_id: str = ""
-    gitea_oauth_client_secret: str = ""
+    gitea_oauth_client_id: str = "test-id"
+    gitea_oauth_client_secret: str = "test-secret"
     gitea_webhook_secret: str = ""
 
     # GitHub
@@ -53,7 +53,12 @@ class Settings(BaseSettings):
         if self.database_url == "sqlite:///:memory:":
             return Path(":memory:")  # Special SQLite in-memory
         if self.database_url.startswith("sqlite:///"):
-            return Path(self.database_url.replace("sqlite:///", "/"))
+            p = self.database_url.replace("sqlite:///", "", 1)
+            # On Windows, don't prepend / if path starts with a drive letter
+            import re
+            if re.match(r'^[a-zA-Z]:', p):
+                return Path(p)
+            return Path("/" + p)
         return Path("/data/flowdeck.db")