Keep what the page installed when the server restarts

The page edits a working copy of the flash image on purpose -- the file it was first pointed at may be a real calibration dump -- but a restart pointed at a different image switched to that file's copy instead, and the games installed through the page looked like they had vanished (measured: the app table came back holding an older slot). work/run-webui.ps1 now prefers the working copy the page has been editing, ahead of the original, and the page's own hint says which image it is editing and that the loaded one is never modified. A test asserts the order in the script.

tools/webui.py

@@ -975,7 +975,9 @@ def render_index(scale: int) -> str:
   <div class="fwbar">
     <label class="mini">flash image<input type="file" id="flashfile" accept=".img,.bin"></label>
     <span class="hint">the multi-system slots live in the external flash;
-    write a .bin into one, then press Multiboot</span>
+    write a .bin into one, then press Multiboot. Edits go to a working copy of the image
+    (the file named above), so the one you loaded is never modified — and a restart keeps
+    that copy rather than reverting to the original</span>
   </div>
   <table id="slottable"><tbody></tbody></table>
   </details>