fix: real video fullscreen, and edge navigation arrows

The full-screen button did nothing: WKWebView inside a Tauri window has
element fullscreen disabled outright, so both requestFullscreen() and
WebKit's webkitEnterFullscreen() are inert, and Tauri exposes no switch
for it. Fullscreening the window alone is not the same thing either —
the player's header and footer stay on screen around the picture. The
button now fullscreens the window and hides every piece of chrome, so
the video alone fills the display, with Esc and a hover control to exit.

Prev/next also get large arrows on the left and right edges of the
picture, fading in on hover, alongside the header buttons.

src-tauri/capabilities/default.json

@@ -8,6 +8,8 @@
   "permissions": [
     "core:default",
     "opener:default",
-    "dialog:default"
+    "dialog:default",
+    "core:window:allow-set-fullscreen",
+    "core:window:allow-is-fullscreen"
   ]
 }