refactor: replace calloc with g_new0 for struct allocations

Migrate structure allocations from calloc to glibs g_new0 macro to
improve type safety and memory robustness.

* Type Safety: The macro takes the type name directly, ensuring the
  allocated size always matches the pointer type.
* Static Analysis: It guarantees a non-NULL return by aborting on
  failure, which silences -fanalyzer warnings regarding potential NULL
  pointer dereferences.
* Readability: Removes redundant sizeof() calls and is the glib way

src/tools/aesgcm_download.c

@@ -101,7 +101,7 @@ aesgcm_file_get(void* userdata)
 
     // We wrap the HTTPDownload tool and use it for retrieving the ciphertext
     // and storing it in the temporary file previously opened.
-    HTTPDownload* http_dl = calloc(1, sizeof(HTTPDownload));
+    HTTPDownload* http_dl = g_new0(HTTPDownload, 1);
     http_dl->window = aesgcm_dl->window;
     http_dl->worker = aesgcm_dl->worker;
     http_dl->id = strdup(aesgcm_dl->id);

src/tools/autocomplete.c

@@ -62,7 +62,7 @@ static gchar* _search(Autocomplete ac, GList* curr, gboolean quote, search_direc
 Autocomplete
 autocomplete_new(void)
 {
-    return calloc(1, sizeof(struct autocomplete_t));
+    return g_new0(struct autocomplete_t, 1);
 }
 
 void