platform: modularize api/gui, add docs-tests-web foundation, and refresh root config

selective-vpn-gui/api/errors.py

@@ -0,0 +1,20 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Optional
+
+
+@dataclass(frozen=True)
+class ApiError(Exception):
+    """Raised when API call fails (network or non-2xx)."""
+
+    message: str
+    method: str
+    url: str
+    status_code: Optional[int] = None
+    response_text: str = ""
+
+    def __str__(self) -> str:
+        code = f" ({self.status_code})" if self.status_code is not None else ""
+        tail = f": {self.response_text}" if self.response_text else ""
+        return f"{self.message}{code} [{self.method} {self.url}]{tail}"