UEVR: inject with Frame Control's own script instead of UEVR's GUI injector

UEVR's .NET injector, driven with XSendEvent, injected in only 6 of 8 cold
starts on the Frame: in bad sessions every new injector ignored input or never
showed a window. start() now runs frame_inject.py under python.org's
embeddable Python (pinned SHA-256) in the game's Wine session, repeating the
frontend's own Inject steps: nullify VR plugins, load openvr_api.dll, set
Frontend_RequestedRuntime, load UEVRBackend.dll. Verified 9/9 cold starts in
33-35 s and a clean UI install/play/remove cycle. No .NET download any more
(47 MB installed instead of about 190 MB).

Also from the SWE-2 Max review: one mod action at a time (flock), a download
size cap, errors always reported as JSON, a receipt can't name another app's
prefix, 0 frames counts as an answer, and a longer server timeout for start.

Part of #26.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ui/index.html

@@ -670,8 +670,9 @@
   <h2 id="modTitle">Play in VR with UEVR</h2>
   <p id="modState" class="sub"></p>
   <p class="sub">UEVR by praydog turns many Unreal Engine games into VR games. Frame Control downloads
-    the official UEVR 1.05 release and Microsoft's .NET 6 runtime it needs (about 70 MB), checks their hashes,
-    and keeps them in this game's Proton prefix, not the game's folder. Remove takes them out again.</p>
+    the official UEVR 1.05 release and python.org's Windows Python, which it uses to load UEVR into the game
+    (about 20 MB), checks their hashes, and keeps them in this game's Proton prefix, not the game's folder.
+    Remove takes them out again.</p>
   <p class="sub">Tested on the Frame with one game so far. Whether a game looks and plays right in VR varies;
     UEVR's own menu in the headset has the per-game settings.</p>
   <div class="row rep-actions"><span class="sub" id="modMsg"></span><span class="spacer"></span>