Use full page width with a responsive link grid

The layout was capped at 1200px and listed links in a single column.
Drop the cap and lay the cards out in a grid: three per row on wide
screens, two from 1000px, one below that.

To make the narrower cards work, shorten the edit action to an icon
button so the title and URL keep their room, and let the edit form's
field and button rows wrap.

Also fixes pre-existing horizontal overflow on phone widths by
stacking the sidebar above the content and letting the add-link form
wrap. Grid tracks use minmax(0, 1fr) so cards can shrink below their
content's intrinsic width instead of pushing the page wider.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/templates/_link_card_edit.html

@@ -18,12 +18,12 @@
       <textarea name="summary" rows="3" style="width:100%; background:var(--panel2); border:1px solid var(--border); color:var(--text); border-radius:8px; padding:10px 12px; font-size:.95rem; font-family:inherit;">{{ link.summary }}</textarea>
     </div>
 
-    <div style="display:flex; gap:12px;">
-      <div class="field" style="flex:1;">
+    <div style="display:flex; gap:12px; flex-wrap:wrap;">
+      <div class="field" style="flex:1 1 160px;">
         <label>Kategorie</label>
         <input type="text" name="category" value="{{ link.category }}">
       </div>
-      <div class="field" style="flex:1;">
+      <div class="field" style="flex:1 1 160px;">
         <label>Hersteller / Quelle</label>
         <input type="text" name="manufacturer" value="{{ link.manufacturer }}">
       </div>
@@ -34,7 +34,7 @@
       <input type="text" name="tags" value="{{ link.tag_list | join(', ') }}">
     </div>
 
-    <div style="display:flex; justify-content:space-between; gap:8px; margin-top:4px;">
+    <div style="display:flex; justify-content:space-between; gap:8px; margin-top:4px; flex-wrap:wrap;">
       <div style="display:flex; gap:8px;">
         <button type="submit">Speichern</button>
         <button type="button" class="ghost"