feat(search): Domain-Filter als Dropdown im Suchfeld

Links im großen Suchfeld ein Slider-Icon mit Badge („Alle" oder „2/5"),
das ein Dropdown-Menü mit allen Whitelist-Domains als Checkboxen öffnet.
Auswahl wird per localStorage persistiert und gilt global — Header-Such-
Dropdown konsumiert den gleichen Store und sendet den domains-Parameter
bei jedem Fetch mit.

Leere Menge heißt „alle aktiv", damit neu vom Admin freigeschaltete
Domains automatisch dabei sind. Aktive Auswahl landet als explizite
Intersection mit der Whitelist serverseitig.

- searchLocal nimmt jetzt optional string[] domains → `source_domain IN (…)`.
- searchWeb nimmt jetzt opts.domains → site:-Filter auf die Auswahl
  eingeschränkt. Nicht-Whitelist-Einträge werden ignoriert.
- API-Endpoints: `?domains=a.de,b.de`.
- Neuer Client-Store $lib/client/search-filter.svelte.ts.
- Neue Komponente $lib/components/SearchFilter.svelte (mobile-tauglich,
  44px Touch-Targets, Badge auf engen Screens versteckt).

Home-Seite re-runt die Suche bei Filter-Änderung automatisch (150ms debounce),
ohne dass der User neu tippen muss.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/components/SearchFilter.svelte

@@ -0,0 +1,246 @@
+<script lang="ts">
+  import { SlidersHorizontal, Check, ChevronDown } from 'lucide-svelte';
+  import { searchFilterStore } from '$lib/client/search-filter.svelte';
+
+  let open = $state(false);
+  let container: HTMLElement | undefined = $state();
+
+  function toggle() {
+    open = !open;
+  }
+
+  function handleClickOutside(e: MouseEvent) {
+    if (container && !container.contains(e.target as Node)) open = false;
+  }
+
+  function handleKey(e: KeyboardEvent) {
+    if (e.key === 'Escape' && open) open = false;
+  }
+
+  $effect(() => {
+    if (open) {
+      document.addEventListener('click', handleClickOutside);
+      document.addEventListener('keydown', handleKey);
+      return () => {
+        document.removeEventListener('click', handleClickOutside);
+        document.removeEventListener('keydown', handleKey);
+      };
+    }
+  });
+
+  function checked(domain: string): boolean {
+    // Leere Menge (default "alle") heißt: jeder Eintrag ist implizit aktiv.
+    if (searchFilterStore.active.size === 0) return true;
+    return searchFilterStore.active.has(domain);
+  }
+
+  function onToggleDomain(domain: string) {
+    if (searchFilterStore.active.size === 0) {
+      // Aus "Alle" wird "alle außer dem abgewählten": explizite Liste mit
+      // allen Domains minus der einen.
+      const rest = searchFilterStore.domains
+        .map((d) => d.domain)
+        .filter((d) => d !== domain);
+      searchFilterStore.active = new Set(rest);
+      searchFilterStore.persist();
+      return;
+    }
+    searchFilterStore.toggle(domain);
+  }
+</script>
+
+<div class="wrap" bind:this={container}>
+  <button
+    class="trigger"
+    class:filtered={searchFilterStore.isFiltered}
+    type="button"
+    aria-label="Suchfilter"
+    aria-haspopup="menu"
+    aria-expanded={open}
+    onclick={toggle}
+  >
+    <SlidersHorizontal size={16} strokeWidth={2} />
+    <span class="badge">{searchFilterStore.label}</span>
+    <ChevronDown size={14} strokeWidth={2} />
+  </button>
+
+  {#if open}
+    <div class="menu" role="menu">
+      <div class="menu-head">
+        <span class="head-title">Gefunden auf</span>
+        <button
+          class="quick"
+          type="button"
+          onclick={() => {
+            searchFilterStore.selectAll();
+          }}
+        >
+          Alle
+        </button>
+      </div>
+      {#if searchFilterStore.domains.length === 0}
+        <p class="empty">Keine Domains in der Whitelist.</p>
+      {:else}
+        <ul class="list">
+          {#each searchFilterStore.domains as d (d.id)}
+            {@const isOn = checked(d.domain)}
+            <li>
+              <button
+                class="row"
+                type="button"
+                role="menuitemcheckbox"
+                aria-checked={isOn}
+                onclick={() => onToggleDomain(d.domain)}
+              >
+                <span class="box" class:on={isOn}>
+                  {#if isOn}<Check size={14} strokeWidth={3} />{/if}
+                </span>
+                <span class="dom">{d.display_name ?? d.domain}</span>
+              </button>
+            </li>
+          {/each}
+        </ul>
+      {/if}
+    </div>
+  {/if}
+</div>
+
+<style>
+  .wrap {
+    position: relative;
+    flex-shrink: 0;
+  }
+  .trigger {
+    display: inline-flex;
+    align-items: center;
+    gap: 0.35rem;
+    padding: 0.5rem 0.75rem;
+    background: white;
+    border: 1px solid #cfd9d1;
+    border-radius: 10px;
+    color: #2b6a3d;
+    cursor: pointer;
+    font-size: 0.88rem;
+    min-height: 44px;
+    font-family: inherit;
+  }
+  .trigger:hover {
+    background: #f4f8f5;
+  }
+  .trigger.filtered {
+    background: #eaf4ed;
+    border-color: #2b6a3d;
+  }
+  .badge {
+    font-weight: 600;
+  }
+  .menu {
+    position: absolute;
+    top: calc(100% + 0.4rem);
+    left: 0;
+    min-width: 260px;
+    max-width: calc(100vw - 2rem);
+    background: white;
+    border: 1px solid #e4eae7;
+    border-radius: 12px;
+    box-shadow: 0 14px 40px rgba(0, 0, 0, 0.18);
+    z-index: 80;
+    padding: 0.35rem;
+    max-height: 70vh;
+    overflow-y: auto;
+  }
+  .menu-head {
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+    padding: 0.45rem 0.75rem;
+    border-bottom: 1px solid #f0f3f1;
+  }
+  .head-title {
+    font-size: 0.78rem;
+    font-weight: 700;
+    color: #666;
+    text-transform: uppercase;
+    letter-spacing: 0.04em;
+  }
+  .quick {
+    background: transparent;
+    border: 0;
+    color: #2b6a3d;
+    font-size: 0.88rem;
+    cursor: pointer;
+    padding: 0.25rem 0.4rem;
+    border-radius: 6px;
+  }
+  .quick:hover {
+    background: #eaf4ed;
+  }
+  .list {
+    list-style: none;
+    padding: 0.2rem 0;
+    margin: 0;
+    display: flex;
+    flex-direction: column;
+    gap: 0.1rem;
+  }
+  .row {
+    display: flex;
+    align-items: center;
+    gap: 0.7rem;
+    width: 100%;
+    background: transparent;
+    border: 0;
+    padding: 0.65rem 0.75rem;
+    border-radius: 8px;
+    cursor: pointer;
+    font-size: 0.95rem;
+    color: #1a1a1a;
+    text-align: left;
+    min-height: 44px;
+    font-family: inherit;
+  }
+  .row:hover {
+    background: #f4f8f5;
+  }
+  .box {
+    width: 20px;
+    height: 20px;
+    border: 1.5px solid #cfd9d1;
+    border-radius: 5px;
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    color: white;
+    background: white;
+    flex-shrink: 0;
+  }
+  .box.on {
+    background: #2b6a3d;
+    border-color: #2b6a3d;
+  }
+  .dom {
+    flex: 1;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    white-space: nowrap;
+  }
+  .empty {
+    padding: 0.8rem 0.75rem;
+    color: #888;
+    font-size: 0.9rem;
+    margin: 0;
+  }
+  @media (max-width: 520px) {
+    .trigger {
+      padding: 0.5rem 0.55rem;
+      font-size: 0.82rem;
+    }
+    .badge {
+      display: none;
+    }
+    .menu {
+      left: -0.25rem;
+      min-width: calc(100vw - 2rem);
+    }
+  }
+</style>