docs: the phone client, and the Glyph traps worth not re-learning

The android README still described the app as unbuilt and pointed `glyph/libs/`
at a path that does not exist. It now covers what the app is made of, how to get
the SDK, and the three Glyph behaviours that cost an afternoon — two of which
fail with no exception and no log at all.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

desktop/kadence_desktop/api.py

@@ -51,11 +51,19 @@ class Api:
                 return json.loads(body) if body else None
         except urllib.error.HTTPError as exc:
             detail = _detail(exc)
+            if exc.code == 401:
+                hint = (
+                    "Set the API token in Settings → Backend."
+                    if self.token
+                    else "No API token configured — add it in Settings → Backend."
+                )
+                raise ApiError(f"{detail} {hint}", exc.code) from exc
             raise ApiError(detail, exc.code) from exc
         except (urllib.error.URLError, TimeoutError) as exc:
             raise ApiError(
                 f"Cannot reach the backend at {self.base_url} ({exc}). "
-                "Is `docker compose up` running?"
+                "Check the server URL in Settings → Backend, or is "
+                "`docker compose up` running locally?"
             ) from exc
         except json.JSONDecodeError as exc:
             raise ApiError(f"The backend sent something that is not JSON: {exc}") from exc