refactor(person): fold year range into section headings, remove standalone stats bar

The floating stats bar was visually disconnected and showed a combined document
count already visible from the per-section badges. Replaced it with a year range
shown inline next to each section heading (e.g. "Gesendete Dokumente · 12 · 1921–1945"),
making the range contextually relevant per direction.

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

frontend/src/routes/persons/[id]/+page.svelte

@@ -24,15 +24,18 @@
 
     const allDocuments = $derived([...sentDocuments, ...receivedDocuments]);
 
-    const docStats = $derived(() => {
-        const dated = allDocuments.filter(d => d.documentDate);
-        const years = dated.map(d => parseInt(d.documentDate!.substring(0, 4)));
-        return {
-            total: allDocuments.length,
-            minYear: years.length ? Math.min(...years) : null,
-            maxYear: years.length ? Math.max(...years) : null,
-        };
-    });
+    function yearRange(docs: typeof sentDocuments) {
+        const years = docs
+            .filter(d => d.documentDate)
+            .map(d => parseInt(d.documentDate!.substring(0, 4)));
+        if (!years.length) return null;
+        const min = Math.min(...years);
+        const max = Math.max(...years);
+        return min === max ? `${min}` : `${min} – ${max}`;
+    }
+
+    const sentYearRange = $derived(yearRange(sentDocuments));
+    const receivedYearRange = $derived(yearRange(receivedDocuments));
 
     const coCorrespondents = $derived(() => {
         const freq = new Map<string, { id: string; name: string; count: number }>();
@@ -327,21 +330,6 @@
     </div>
     {/if}
 
-    <!-- Document Statistics Bar -->
-    {#if docStats().total > 0}
-    <div class="mb-8 px-4 py-3 bg-brand-sand/30 rounded-sm flex items-center gap-2 font-sans text-sm text-brand-navy/70">
-        <span>{docStats().total} Dokumente</span>
-        {#if docStats().minYear !== null}
-            <span class="text-brand-mint">·</span>
-            {#if docStats().minYear === docStats().maxYear}
-                <span>{docStats().minYear}</span>
-            {:else}
-                <span>{docStats().minYear} – {docStats().maxYear}</span>
-            {/if}
-        {/if}
-    </div>
-    {/if}
-
     <!-- Sort control -->
     {#if allDocuments.length > 0}
     <div class="flex justify-end mb-4">
@@ -362,6 +350,9 @@
             <span class="bg-brand-navy text-white text-xs font-bold px-2 py-1 rounded-full">
                 {sentDocuments.length}
             </span>
+            {#if sentYearRange}
+                <span class="text-xs font-sans text-gray-400">{sentYearRange}</span>
+            {/if}
         </div>
 
         {#if sentDocuments.length === 0}
@@ -423,6 +414,9 @@
             <span class="bg-brand-navy text-white text-xs font-bold px-2 py-1 rounded-full">
                 {receivedDocuments.length}
             </span>
+            {#if receivedYearRange}
+                <span class="text-xs font-sans text-gray-400">{receivedYearRange}</span>
+            {/if}
         </div>
 
         {#if receivedDocuments.length === 0}