Restructure Settings into tabbed sections

Split the long four-panel Settings page into hash-driven tabs (Familj,
Saldon, Lediga dagar, Allmänt) with a sticky side nav on desktop and a
scrollable tab bar on mobile. Each card saves on its own, the profile
form is prefilled from the member, and save feedback is a fixed toast.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/styles/components.css

@@ -755,10 +755,6 @@
   min-height: 100px;
 }
 
-.settings-page > .alert {
-  margin-bottom: var(--sp-4);
-}
-
 /* Fieldsets */
 fieldset {
   border: 1px solid var(--c-border);
@@ -811,3 +807,11 @@ legend {
 .parent-profile-input__field {
   max-width: 400px;
 }
+
+/* Stacked fields — the same gap as other stacked forms, so a label never
+ * sits tight under the select above it. */
+.parent-profile-input {
+  display: flex;
+  flex-direction: column;
+  gap: var(--sp-4);
+}