security: announce a fresh EIP-6963 provider UUID per page load (closes #398)

The provider UUID was generated once, stored in extension storage, and
announced verbatim to every page on every load and across restarts, so any
site — connected or not — could read a stable cross-site, cross-session
identifier for the install: a supercookie contradicting the "no tracking"
promise. EIP-6963 wants a fresh UUIDv4 per announcement instead.

inpage.js now announces a per-load crypto.randomUUID() and persists nothing;
the eip6963Uuid storage key and the AUTISTMASK_PROVIDER_UUID content-script
message are removed. That key was a standalone top-level storage entry, never
part of the versioned autistmask profile, so stateSchema.js and the
persisted-field harness are untouched and no existing profile is affected.

A jest test asserts two loads announce different UUIDv4s and that one load
reuses a single UUID across re-announcements.

Model: opus-4-8

src/content/inpage.js

@@ -204,7 +204,14 @@
                 "</svg>",
         );
 
-    let providerUuid = crypto.randomUUID(); // fallback until real UUID arrives
+    // EIP-6963 wants a fresh UUIDv4 per page load — it identifies one
+    // announcement, so a provider can be told apart from another instance of
+    // itself in the same page. It is generated here and never stored:
+    // announcing one persisted value to every site, on every load and across
+    // restarts, turned it into a stable cross-site, cross-session tracking
+    // identifier any page could read
+    // (https://git.eeqj.de/sneak/AutistMask/issues/398).
+    const providerUuid = crypto.randomUUID();
 
     function buildProviderInfo() {
         return {
@@ -226,16 +233,6 @@
         );
     }
 
-    // Listen for the persisted UUID from the content script
-    function onProviderUuid(event) {
-        if (event.source !== window) return;
-        if (event.data?.type !== "AUTISTMASK_PROVIDER_UUID") return;
-        window.removeEventListener("message", onProviderUuid);
-        providerUuid = event.data.uuid;
-        announceProvider();
-    }
-    window.addEventListener("message", onProviderUuid);
-
     window.addEventListener("eip6963:requestProvider", announceProvider);
     announceProvider();