board: spinner during first load instead of "no cards found"

An empty board on first load blurting "no cards found" was alarming
on a board the user knew had cards. Track a cardsLoaded flag that
flips on the first /api/cards (meta or full) response — or on a
cached-cards hydration — and show a centred spinner placeholder
while it's still false. The "no cards found" copy only shows once
we've actually heard back and the set is genuinely empty.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/js/app.js

@@ -45,6 +45,7 @@ window.openFindBar = openFindBar;
 window._reloadAll = async () => {
     await loadBuckets();
     state.cards = await api('api/cards') || [];
+    state.cardsLoaded = true;
     renderBoard();
     refreshCalendar();
     // Let plugins / modules react to a full card reload (detail view checks
@@ -86,10 +87,14 @@ function setCachedCards(cards) {
 }
 
 async function loadCards() {
-    // Phase 0: Show cached cards instantly (from previous session)
+    // Phase 0: Show cached cards instantly (from previous session).
+    // A hit here is enough to flip cardsLoaded — fresh meta will
+    // overwrite state.cards a moment later, but the spinner→content
+    // transition already happened.
     const cached = getCachedCards();
     if (cached.length) {
         state.cards = cached.map(c => ({ ...c, body: null }));
+        state.cardsLoaded = true;
         updateSortUI();
         renderBoard();
     }
@@ -97,6 +102,7 @@ async function loadCards() {
     // Phase 1: Load metadata → update board with fresh titles/tags/dates
     const meta = await api('api/cards/meta');
     if (!meta) return;
+    state.cardsLoaded = true;
     state.cards = meta.map(c => ({ ...c, body: null }));
     setCachedCards(meta);
     updateSortUI();
@@ -158,7 +164,7 @@ importFile?.addEventListener('change', async e => {
         if (!res.ok) throw new Error(text || `HTTP ${res.status}`);
         const data = JSON.parse(text);
         if (data.error) throw new Error(data.error);
-        await loadBuckets(); state.cards = await api('api/cards'); renderBoard();
+        await loadBuckets(); state.cards = await api('api/cards'); state.cardsLoaded = true; renderBoard();
     } catch (err) {
         await showDialog({ title: 'import failed', message: err.message || 'Unknown error', danger: true, okLabel: 'OK', cancelLabel: '' });
     }