fix(web): scripts can open files in browser storage; stored footage isn't reported missing

In the browser editor a .jsx script couldn't open any file, even one the
page holds next to the project: File.open("r") returned false with "File
not found" and File.exists was false. The footage check after File > Open
also marked imported media as missing.

Both asked whether a file exists on the local disk (std::fs), which has no
files on wasm32:
- scripts read files through Session.services but checked existence with
  std::fs (File.open's access check, File.exists, Folder.exists); they now
  ask services.exists / services.is_dir, after the folder check;
- WebServices kept the default exists (std::fs); it now looks in the file
  table.

Desktop and CLI are unchanged (FsServices keeps the std::fs defaults).
Tests: script tests_services (a host with in-memory files), web smoke
step hostFiles (a script reads a stored file, footage check finds stored
media).

docs/web.md

@@ -413,7 +413,8 @@ no npm packages): load, the GPU path (WebGPU → GPU compositor, viewer on the G
 in the viewer, `render.frame`, Render Queue GIF and PNG sequence (`.zip`) downloads, a background
 render in a worker while measuring the page's event-loop gaps (must stay under 400 ms) and its
 progress, the AudioContext starting on a (simulated) key press and a tone playing through it
-with the meters moving, Save As and re-opening the saved project, persistence across a reload
+with the meters moving, Save As and re-opening the saved project, a script opening a file from
+browser storage and the footage check finding stored media, persistence across a reload
 (session snapshot, a stored project, imported media, a setting, Open Recent) and an offline
 reload through the service worker. It writes screenshots and `report.json`: