feat: only show the migration dialog on the ChromeOS webapp

phcode.dev now loads silently for everyone else. The dialog was going up on
every boot of the legacy origin, and outside the ChromeOS webapp it was not
earning that: the origin still works, and anyone who does open the new site
has their data pulled across automatically, so there is no action the modal
has to get out of the user.

The gate sits in main.js next to the isTestWindow guard, where the decision
about which half of the flow runs already lives. sunset-dialog.js is
unchanged apart from its header, so widening the audience later is a one
line revert.

No test covers this. main.js has no exports and _initLegacyOrigin already
short circuits in a test window, so the gate is unreachable from a spec
without moving the predicate into constants.js, where the test would only
restate the implementation.

Author: abose

src/extensionsIntegrated/MigrateAssist/main.js

@@ -22,7 +22,8 @@
  * Entry point for the one time move off the legacy web origin onto web.phcode.dev.
  *
  * Which half runs depends purely on which origin this window is:
- *  - on the origin being retired, announce the move once per boot (sunset-dialog);
+ *  - on the origin being retired, announce the move once per boot (sunset-dialog), currently only
+ *    on the ChromeOS webapp;
  *  - on the new origin, quietly check whether anything is left behind and pull it across
  *    (migrator), plus register the Help menu entry that lets the user ask for it again later.
  *
@@ -45,7 +46,10 @@ define(function (require, exports, module) {
         Migrator = require("./migrator");
 
     function _initLegacyOrigin() {
-        if (Phoenix.isTestWindow) {
+        // For now the announcement only goes up on the ChromeOS webapp. Everywhere else the legacy
+        // origin stays silent: it keeps working, and the data of anyone who does open the new site
+        // is pulled across automatically, so there is nothing the dialog has to make them do.
+        if (Phoenix.isTestWindow || !Phoenix.browser.isChromeOS) {
             return;
         }
         SunsetDialog.show();

src/extensionsIntegrated/MigrateAssist/sunset-dialog.js

@@ -19,7 +19,8 @@
  */
 
 /**
- * The "we are moving" dialog, shown on the origin being retired on every boot.
+ * The "we are moving" dialog, shown on every boot of the origin being retired. main.js decides who
+ * gets it; for now that is the ChromeOS webapp only.
  *
  * It states that the move is happening, with no countdown and no cutoff date: the move is under way
  * now, and naming a date we might not hold to would be worse than naming none.
@@ -87,7 +88,7 @@ define(function (require, exports, module) {
     }
 
     /**
-     * Shows the dialog. Called once per boot on the legacy origin; dismissing it is per boot only,
+     * Shows the dialog. Called once per boot by main.js; dismissing it is per boot only,
      * so the user is reminded again next time rather than being able to silence it permanently.
      */
     function show() {