fix: count the grid in the post header, and qualify the Instagram claim

The header rendered allPosts.length, which is pre-dedupe — 0ct0ber19
showed "303 posts" over a 300-tile grid. Instagram's counter equals its
grid, so count the grid.

Also correct CLAUDE.md. v1.7.0 claimed the grid holds everything "as on
Instagram"; Instagram actually includes a reel in the grid only when the
creator shared it to feed, per post. Measured live: official_artms has
21 reels in its first 34 grid tiles, 0ct0ber19 has 1 in 214. Archives
carry no such flag, so showing everything approximates the behaviour
rather than reproducing it.

Records the DOM trap that caused the wrong reading in the first place:
grid reels link to /reel/<code>/, not /<user>/p/<code>/.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "instaarchive-viewer",
   "private": true,
-  "version": "1.7.0",
+  "version": "1.7.1",
   "type": "module",
   "scripts": {
     "dev": "vite --port=3000 --host=0.0.0.0",