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/modules/state.js

@@ -6,6 +6,11 @@ export const state = {
     // Data
     cards: [],
     buckets: [],
+    // True once we've got a real response from api/cards (or the
+    // cached-cards hydration). Until then the board shows a spinner
+    // instead of the "no cards found" placeholder — an empty board on
+    // first load is an honest "still fetching" state, not a real empty.
+    cardsLoaded: false,
 
     // Filters & view — activeBucket is persisted across reloads via
     // localStorage. Numeric ids are stringified; the special filters