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/new/+page.svelte

@@ -4,15 +4,17 @@ import TagInput from '$lib/components/TagInput.svelte';
 import PersonTypeahead from '$lib/components/PersonTypeahead.svelte';
 import PersonMultiSelect from '$lib/components/PersonMultiSelect.svelte';
 
-export let form;
+let { form } = $props();
 
-let tags: string[] = [];
-let senderId = '';
-let selectedReceivers: { id: string; firstName: string; lastName: string }[] = [];
+let tags: string[] = $state([]);
+let senderId = $state('');
+let selectedReceivers: { id: string; firstName: string; lastName: string }[] = $state([]);
 
-let dateDisplay = '';
-let dateIso = '';
-let dateDirty = false;
+let dateDisplay = $state('');
+let dateIso = $state('');
+let dateDirty = $state(false);
+
+const dateInvalid = $derived(dateDirty && dateDisplay.length > 0 && dateIso === '');
 
 function germanToIso(german: string): string {
 	const match = german.match(/^(\d{2})\.(\d{2})\.(\d{4})$/);
@@ -37,8 +39,6 @@ function handleDateInput(e: Event) {
 	dateIso = germanToIso(formatted);
 	dateDirty = true;
 }
-
-$: dateInvalid = dateDirty && dateDisplay.length > 0 && dateIso === '';
 </script>
 
 <div class="mx-auto max-w-4xl px-4 py-8">
@@ -86,7 +86,7 @@ $: dateInvalid = dateDirty && dateDisplay.length > 0 && dateIso === '';
 						type="text"
 						inputmode="numeric"
 						value={dateDisplay}
-						on:input={handleDateInput}
+						oninput={handleDateInput}
 						placeholder="TT.MM.JJJJ"
 						maxlength="10"
 						class="block w-full rounded border border-gray-300 p-2 text-sm shadow-sm