feat(downloads): add pref to control download popup position independently, p=#11607

* feat(downloads): add pref to control download popup position independently

Add `zen.downloads.icon-popup-position` preference that allows users to
control the download popup/indicator position independently from the
vertical tabs position.

Valid values:
- "follow-tabs" (default): popup appears on same side as vertical tabs
- "left": popup always appears on the left
- "right": popup always appears on the right

This is useful for users who have vertical tabs on the right but prefer
the download indicator to appear on the left side of the screen.

* feat: Convert pref to integers, b=no-bug, c=no-component

---------

Co-authored-by: salmonumbrella <salmonumbrella@users.noreply.github.com>
Co-authored-by: mr. m <mr.m@tuta.com>

prefs/zen/downloads.yaml

@@ -7,3 +7,9 @@
 
 - name: zen.downloads.download-animation-duration
   value: 1000 # ms
+
+- name: zen.downloads.icon-popup-position
+  # 0: Follow tab's position
+  # 1: Left side always
+  # 2: Right side always
+  value: 0

src/zen/downloads/ZenDownloadAnimation.mjs

@@ -131,6 +131,9 @@ class nsZenDownloadAnimationElement extends HTMLElement {
   }
 
   #areTabsOnRightSide() {
+    const position = Services.prefs.getIntPref('zen.downloads.icon-popup-position', 0);
+    if (position === 1) return false;
+    if (position === 2) return true;
     return Services.prefs.getBoolPref('zen.tabs.vertical.right-side');
   }