feat: add categorized settings tabs (#30)

Six tabs (General, Meals, Budget, Shopping, Calendar, Account) replace
the flat single-page layout. Active tab persists via sessionStorage.
Calendar tab auto-activates on OAuth redirect. Tab bar is sticky.
All labels translated in de/en/es/it/sv.

public/styles/settings.css

@@ -42,6 +42,54 @@
   border: 1px solid var(--color-danger);
 }
 
+/* --------------------------------------------------------
+   Tab-Navigation
+   -------------------------------------------------------- */
+
+.settings-tabs {
+  display: flex;
+  gap: 0;
+  overflow-x: auto;
+  scrollbar-width: none;
+  -webkit-overflow-scrolling: touch;
+  border-bottom: 1px solid var(--color-border);
+  margin-bottom: var(--space-6);
+  position: sticky;
+  top: 0;
+  background: var(--color-background);
+  z-index: 10;
+  padding-top: var(--space-1);
+}
+
+.settings-tabs::-webkit-scrollbar {
+  display: none;
+}
+
+.settings-tab-btn {
+  flex-shrink: 0;
+  padding: var(--space-3) var(--space-4);
+  border: none;
+  border-bottom: 2px solid transparent;
+  background: transparent;
+  color: var(--color-text-secondary);
+  font-size: var(--text-sm);
+  font-weight: var(--font-weight-medium);
+  cursor: pointer;
+  transition: color var(--transition-fast), border-color var(--transition-fast);
+  min-height: 44px;
+  white-space: nowrap;
+  margin-bottom: -1px;
+}
+
+.settings-tab-btn:hover {
+  color: var(--color-text-primary);
+}
+
+.settings-tab-btn--active {
+  color: var(--color-accent);
+  border-bottom-color: var(--color-accent);
+}
+
 /* --------------------------------------------------------
    Sections
    -------------------------------------------------------- */