fix(webui): send DELETE when removing keys and adapters

Deleting a gateway key from the admin UI did nothing and reported
"use GET /api/keys": delKey() called api() with an empty options object, so
fetch defaulted to GET and the request landed in the GET branch of
handleKeysAPI. delAdapter() had the identical bug and reported
"adapter code not exposed; edit in UI".

This is the third instance of the same mistake — ab20f1b fixed delSource and
delTemplate, missing these two — so it is now pinned by tests instead of by
review:

  * TestUIAPICallsDeclareMethod walks every api() call in the embedded
    index.html and fails if one passes an options object without a method
    (an AbortSignal-only read is allowed, being a deliberate GET).
  * TestUIDeleteHelpersUseDelete / TestUIMutatingHelpersUseWriteMethods pin the
    verb of each removal and write helper by name.
  * TestKeyDeleteRoundTrip covers create -> DELETE -> gone -> second DELETE is a
    clean 404, and TestCannotDeleteOwnKey keeps the lockout guard.

The 404 bodies for GET /api/keys/{key} and GET /api/adapters/{name} now name the
verb to use ("DELETE /api/keys/{key} to remove"), because that message is what a
mis-methoded client actually shows its user; "use GET /api/keys" read as though
the caller had done nothing wrong.

delSource's indentation, broken by ab20f1b, is also straightened out.

internal/gateway/api.go

@@ -32,7 +32,11 @@ func (g *Gateway) handleAdaptersAPI(w http.ResponseWriter, r *http.Request) {
 			writeJSON(w, http.StatusOK, map[string]interface{}{"adapters": g.core.ListAdapters()})
 			return
 		}
-		writeError(w, http.StatusNotFound, "not_found", "adapter code not exposed; edit in UI")
+		// A GET on a specific adapter is almost always a client that meant to
+		// DELETE it but let fetch default to GET; say so instead of only
+		// reporting that the code is not exposed.
+		writeError(w, http.StatusNotFound, "not_found",
+			"adapter code not exposed; edit in UI (to remove it use DELETE /api/adapters/"+path+")")
 	case http.MethodPost:
 		var p adapterPayload
 		if err := json.NewDecoder(r.Body).Decode(&p); err != nil {