refactor: migrate all Svelte components from Svelte 4 to Svelte 5 runes

- Replace `export let` with `$props()` and `$bindable()` across all components
- Replace `$:` reactive statements with `$derived()` and `$effect()`
- Replace `createEventDispatcher` with callback props (e.g. `onchange`)
- Replace `on:event` directives with inline event handlers (`onclick`, `oninput`, etc.)
- Replace `<slot />` with `{@render children()}` in layout
- Use `untrack()` for SSR-safe $state initialization from reactive props
- Replace `blur` + `setTimeout` anti-pattern in TagInput with `clickOutside` action
- Fix `page` store usage in layout to use `$app/state` directly
- 0 errors, 0 warnings after svelte-check

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

frontend/src/routes/documents/[id]/edit/+page.svelte

@@ -3,14 +3,14 @@
     import TagInput from '$lib/components/TagInput.svelte';
     import PersonTypeahead from '$lib/components/PersonTypeahead.svelte';
     import PersonMultiSelect from '$lib/components/PersonMultiSelect.svelte';
+    import { untrack } from 'svelte';
 
-    export let data;
-    export let form;
+    let { data, form } = $props();
 
-    let { document: doc } = data;
-    let tags = doc.tags ? doc.tags.map((t: any) => t.name) : [];
-    let senderId = doc.sender?.id ?? '';
-    let selectedReceivers = doc.receivers ?? [];
+    let { document: doc } = untrack(() => data);
+    let tags = $state(doc.tags ? doc.tags.map((t: { name: string }) => t.name) : []);
+    let senderId = $state(doc.sender?.id ?? '');
+    let selectedReceivers = $state(doc.receivers ?? []);
 
     function isoToGerman(iso: string): string {
         if (!iso || !/^\d{4}-\d{2}-\d{2}$/.test(iso)) return '';
@@ -25,9 +25,11 @@
         return `${y}-${m}-${d}`;
     }
 
-    let dateDisplay = isoToGerman(doc.documentDate ?? '');
-    let dateIso = doc.documentDate ?? '';
-    let dateDirty = false;
+    let dateDisplay = $state(isoToGerman(doc.documentDate ?? ''));
+    let dateIso = $state(doc.documentDate ?? '');
+    let dateDirty = $state(false);
+
+    const dateInvalid = $derived(dateDirty && dateDisplay.length > 0 && dateIso === '');
 
     function handleDateInput(e: Event) {
         const input = e.target as HTMLInputElement;
@@ -45,8 +47,6 @@
         dateIso = germanToIso(formatted);
         dateDirty = true;
     }
-
-    $: dateInvalid = dateDirty && dateDisplay.length > 0 && dateIso === '';
 </script>
 
 <div class="max-w-4xl mx-auto py-8 px-4">
@@ -84,7 +84,7 @@
                         type="text"
                         inputmode="numeric"
                         value={dateDisplay}
-                        on:input={handleDateInput}
+                        oninput={handleDateInput}
                         placeholder="TT.MM.JJJJ"
                         maxlength="10"
                         class="block w-full rounded border-gray-300 shadow-sm p-2 border text-sm