fix: read xz sidecars by buffer, page carousel with arrows, stop backdrop flash

Instaloader metadata was silently lost
Every .json.xz failed with "Failed to fetch" during a scan, though the same URL
fetched fine on its own. RemoteArchiveFile.stream() started a fetch, piped the
body into a TransformStream and returned the readable immediately — nothing
caught a fetch rejection, and the decompressor stops reading at the end of the
xz member, so the response body was never drained or cancelled. Across ~190
sidecars that exhausted the connection pool.

Everything Instaloader archives carry lives in those files, so the failure was
invisible but total. rivvsofficial reported 188 posts, no stories, 0 followers
and a placeholder bio; it now reports 68 posts, 120 stories, 10,337 followers
and the real name, bio and link — 68 + 120 = 188, matching the sidecars exactly
(106 GraphStoryVideo + 14 GraphStoryImage = 120).

These sidecars are a few KB, so they are now read into memory before
decompressing. stream() was left unused by that change and is removed from the
interface and both implementations rather than kept as a trap.

Arrow keys page the carousel
They moved between posts, which contradicted the arrows drawn on the carousel
itself. Arrows now page slides; , and . move between posts, alongside the side
buttons.

Backdrop cross-fade
AnimatePresence had no exit variant, so the outgoing scan backdrop was removed
instantly while its replacement faded in over 1.5s, exposing the pale page
behind it as a white flash. Layers now stack: the outgoing image holds full
opacity until covered, and the 0.4 moved onto the group so overlapping layers
don't darken as they cross. Measured over a real scan: 152 cross-fades with a
layer always opaque, except the opening fade-in where nothing is underneath.

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": "1.5.1",
+  "version": "1.6.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "instaarchive-viewer",
-      "version": "1.5.1",
+      "version": "1.6.0",
       "dependencies": {
         "@tailwindcss/vite": "^4.1.14",
         "@vitejs/plugin-react": "^5.0.4",