Refactor watch history to use monthly buckets (v3)

Refactors `js/nostr/watchHistory.js` to store and publish watch history items in monthly buckets (`YYYY-MM`) instead of using chunk splitting. Updates storage version to 3, migrates existing data, and simplifies fetch/merge logic to handle monthly records. Removes legacy chunk rebalancing code. Updates tests to verify monthly bucketing behavior.

package.json

@@ -29,7 +29,7 @@
     "cross-env": "^7.0.3",
     "esbuild": "^0.25.10",
     "jsdom": "^24.0.0",
-    "nostr-tools": "^2.17.0",
+    "nostr-tools": "^2.19.4",
     "pixelmatch": "^5.3.0",
     "pngjs": "^7.0.0",
     "postcss": "^8.4.41",