fix: keep post nav arrows outside the modal and stop scroll chaining

The prev/next arrows are fixed to the viewport edges while the modal grows to
fill the available width, so below roughly 1200px the modal slid underneath
them and a white chevron landed on the white caption panel — invisible until
hovered. The overlay now reserves a horizontal gutter (md:px-16 lg:px-24) so
the arrows always sit outside the modal, and they get a solid white pill with a
dark chevron so they read against anything behind them. Verified clearing the
modal at 768, 1024, 1440 and 1920px.

The caption sidebar shrinks to w-80 at md so the narrower modal doesn't squeeze
the media pane.

Also adds overscroll-contain to the overlay: the wheel previously chained
through to the post grid behind it, scrolling the background while a post was
open.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011uBWhwV3wFQ5MBCcMHHem7

package.json

@@ -1,7 +1,7 @@
 {
   "name": "instaarchive-viewer",
   "private": true,
-  "version": "1.3.2",
+  "version": "1.3.3",
   "type": "module",
   "scripts": {
     "dev": "vite --port=3000 --host=0.0.0.0",