refactor(consolidate): IconButton + Badge-solid + @layer base + Massen-Migration

E: app.css Base-h1–h6 in @layer base → text-*!-Hacks in Titeln entfernt.
Neue Atoms: IconButton (runde Icon-Buttons), Badge solid-Variante + uppercase.

Migrationen (6 parallele Pässe):
- IconButton: 19 runde Nav/Close-Buttons (Carousel, Galleries, QuoteCarousel,
  PostOverview, Organisations, ImageModal).
- Badge: Kategorie/Status-Pills (PostCard, BlogOverview, kalender, Comments,
  CalendarBlock, FilesBlock, SearchableText, Stellingnahme).
- ui-Atome: Formular-Inputs (Comments, termin-melden, Stellingnahme) → TextInput/
  Textarea/Checkbox/SearchField; dunkle NewsletterInline bewusst raw.
- Button-Atom: ~20 inline Buttons in StellingnahmeGeneratorBlock.
- Radius-Sweep: Cards→rounded-lg (viele via .card-surface), Buttons/Inputs→rounded-md,
  Chips→rounded-full; StrommixBlock/Calendar/Files/Adressbuch u.a.

svelte-check 0 errors, voller Prod-Build grün.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/components/IconButton.svelte

@@ -0,0 +1,56 @@
+<script lang="ts">
+  /**
+   * IconButton — runder Icon-only Button (Carousel-Pfeile, Modal-Close, …).
+   * Die eine Quelle für kreisförmige Icon-Buttons. `label` = aria-label (Pflicht).
+   * variant: surface (weiß, Border, Shadow — z.B. Slider-Pfeile) · ghost (transparent).
+   */
+  import Icon from "@iconify/svelte";
+  import "$lib/iconify-offline";
+
+  type Size = "sm" | "md";
+  type Variant = "surface" | "ghost";
+
+  let {
+    icon,
+    label,
+    href = undefined,
+    onclick = undefined,
+    size = "md",
+    variant = "surface",
+    disabled = false,
+    class: className = "",
+    ...rest
+  }: {
+    icon: string;
+    label: string;
+    href?: string;
+    onclick?: (e: MouseEvent) => void;
+    size?: Size;
+    variant?: Variant;
+    disabled?: boolean;
+    class?: string;
+    [key: string]: unknown;
+  } = $props();
+
+  const sizeClass: Record<Size, string> = { sm: "size-7", md: "size-9" };
+  const iconClass: Record<Size, string> = { sm: "size-4", md: "size-5" };
+  const variantClass: Record<Variant, string> = {
+    surface:
+      "border border-stein-200 bg-white text-stein-700 shadow-md hover:bg-stein-50",
+    ghost: "text-stein-600 hover:bg-stein-100",
+  };
+
+  const cls = $derived(
+    `inline-flex shrink-0 items-center justify-center rounded-full transition-colors disabled:cursor-not-allowed disabled:opacity-50 ${sizeClass[size]} ${variantClass[variant]} ${className}`,
+  );
+</script>
+
+{#if href && !disabled}
+  <a {href} class={cls} aria-label={label} {...rest}>
+    <Icon icon={icon} class={iconClass[size]} aria-hidden="true" />
+  </a>
+{:else}
+  <button type="button" {disabled} {onclick} class={cls} aria-label={label} {...rest}>
+    <Icon icon={icon} class={iconClass[size]} aria-hidden="true" />
+  </button>
+{/if}