feat: fit full-view media to the viewport and play with sound

Media in the post modal used w-full/h-auto, so a portrait video or image grew
taller than the screen (a 720x1280 reel rendered 768x1365 in a 786px viewport)
and forced the modal to scroll. Full view now caps height to the viewport minus
the modal's own padding. Video sizes to its own aspect within the cap so a
portrait clip isn't letterboxed edge to edge; images keep filling the modal
width and only gain a height ceiling.

Opening the modal or a story reel is a user gesture, so playback now starts
unmuted and only falls back to muted if the browser actually refuses the
play() promise — previously it always started muted, and the earlier
muted-by-default fix meant a blocked video could stall the story progress bar.

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

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "instaarchive-viewer",
-  "version": "1.3.1",
+  "version": "1.3.2",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "instaarchive-viewer",
-      "version": "1.3.1",
+      "version": "1.3.2",
       "dependencies": {
         "@tailwindcss/vite": "^4.1.14",
         "@vitejs/plugin-react": "^5.0.4",