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>

electron/package.json

@@ -1,6 +1,6 @@
 {
   "name": "hyprcrd",
-  "version": "1.9.5",
+  "version": "1.9.6",
   "description": "hyprcrd — self-hosted card manager",
   "main": "main.js",
   "scripts": {