feat(members): implement /members page — Kachel-Ansicht (E2, issue #48)

Backend:
- Rename V006 migration to V026 (avoid conflict with existing V006)
- Migration adds invalidated_at + partial unique index on household_invite

Frontend:
- Toast.svelte — new system component (message + dismiss)
- SegmentedControl.svelte — new system component (options, value, onchange)
- members/+page.server.ts — loads members + active invite
- members/[userId]/+server.ts — DELETE/PATCH proxy
- members/invites/+server.ts — POST (regenerate) proxy
- MemberCard.svelte — tile with avatar, kebab, inline role edit
- RemoveDialog.svelte — confirmation dialog (desktop modal + BottomSheet mobile)
- InviteCard.svelte + InvitePanel.svelte — invite management UI
- MemberGrid.svelte — responsive 4/2-col grid with sorted members
- members/+page.svelte — page composing all components with optimistic updates

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/Toast.svelte

@@ -0,0 +1,31 @@
+<script lang="ts">
+  const { message, visible, ondismiss }: {
+    message: string;
+    visible: boolean;
+    ondismiss?: () => void;
+  } = $props();
+</script>
+
+{#if visible}
+  <div
+    role="status"
+    style="
+      position: fixed;
+      bottom: 24px;
+      right: 24px;
+      z-index: 200;
+      background: var(--color-surface);
+      border: 1px solid var(--color-border);
+      box-shadow: var(--shadow-overlay);
+      border-radius: var(--radius-lg);
+      color: var(--color-text);
+      padding: 12px 16px;
+      display: flex;
+      align-items: center;
+      gap: 12px;
+    "
+  >
+    <span>{message}</span>
+    <button aria-label="Schließen" onclick={ondismiss}>✕</button>
+  </div>
+{/if}