fix(dashboard): hold the run-history height so the guest list keeps its rows

The bottom grid takes its row height from Run History alone, on purpose: the
guest panel fills its cell absolutely and contributes no intrinsic height. That
only works if the history panel's height is constant, and it was not.
.table-scroll capped at 418px instead of holding it, and with no runs at all the
panel rendered its empty state instead of .table-scroll, so the box did not
exist. A fresh install therefore showed one guest row.

The scroller now has a fixed height, the empty state lives inside it so the box
exists in every state, and an expanded run grows past it from a min-height.

frontend/src/dashboard.css

@@ -584,17 +584,20 @@
   color: var(--jn-text);
   background: color-mix(in srgb, var(--jn-accent) 9%, transparent);
 }
-/* 10 rows before it scrolls: 28px header + 10 × 39px (8+8 padding + the 22px status pill
-   + 1px divider) — keep in step if the cell padding or the pill changes. */
+/* 10 rows: 28px header + 10 × 39px (8+8 padding + the 22px status pill + 1px divider) —
+   keep in step if the cell padding or the pill changes. This is a fixed height, not a cap:
+   the bottom grid takes its row height from this panel alone (see .gcell), so letting it
+   shrink on few or zero runs squashed "Last backup per guest" down to a single row. */
 .table-scroll {
   overflow-x: auto;
   overflow-y: auto;
-  max-height: 418px;
+  height: 418px;
 }
-/* An expanded row carries the steps + task log: too tall to read through a 12-row window,
-   so RunHistory lifts the cap for as long as one is open. */
+/* An expanded row carries the steps + task log: too tall to read through a 10-row window,
+   so RunHistory grows past it for as long as one is open — but never below it. */
 .table-scroll.uncapped {
-  max-height: none;
+  height: auto;
+  min-height: 418px;
 }
 .htable {
   width: 100%;

frontend/src/pages/dashboard/RunHistory.tsx

@@ -70,10 +70,13 @@ export function RunHistory({
       </div>
 
       {error && <div className="panel-empty">{t('dashboard.runsError')}</div>}
-      {shown.length === 0 ? (
-        <div className="panel-empty">{t('dashboard.noRuns')}</div>
-      ) : (
-        <div className={`table-scroll${open !== null ? ' uncapped' : ''}`}>
+      {/* The empty state lives *inside* .table-scroll so the box keeps its fixed height:
+          the bottom grid takes its row height from this panel, and a collapsed one leaves
+          "Last backup per guest" one row tall. */}
+      <div className={`table-scroll${open !== null ? ' uncapped' : ''}`}>
+        {shown.length === 0 ? (
+          <div className="panel-empty">{t('dashboard.noRuns')}</div>
+        ) : (
           <table className="htable">
             <thead>
               <tr>
@@ -108,8 +111,8 @@ export function RunHistory({
               ))}
             </tbody>
           </table>
-        </div>
-      )}
+        )}
+      </div>
     </section>
   )
 }