refactor(icons): centralize icon resolution into Paths.resolveIconPath/resolveIconUrl (#1880)

Supersedes #1878. Rather than duplicating the moddedAppId + file path
substitution pattern inline across 8 files, this introduces two
centralized functions in Paths.qml:

- resolveIconPath(iconName): for Quickshell.iconPath() callsites,
  with DesktopService.resolveIconPath() fallback
- resolveIconUrl(iconName): for image://icon/ URL callsites

All consumer files now use one-line calls. When no substitutions are
configured, moddedAppId() returns the original name unchanged (zero
cost), so this has no impact on users who don't use the feature.

Affected components:
- AppIconRenderer (8 lines → 1)
- NotificationCard, NotificationPopup, HistoryNotificationCard
- DockContextMenu, AppsDockContextMenu
- LauncherContent, LauncherTab (×3)

Co-authored-by: odtgit <odtgit@taliops.com>

quickshell/Modules/Notifications/Center/HistoryNotificationCard.qml

@@ -127,7 +127,7 @@ Rectangle {
                     return appIcon;
                 if (appIcon.startsWith("material:") || appIcon.startsWith("svg:") || appIcon.startsWith("unicode:") || appIcon.startsWith("image:"))
                     return "";
-                return Quickshell.iconPath(appIcon, true);
+                return Paths.resolveIconPath(appIcon);
             }
 
             hasImage: hasNotificationImage

quickshell/Modules/Notifications/Center/NotificationCard.qml

@@ -174,7 +174,7 @@ Rectangle {
                     return appIcon;
                 if (appIcon.startsWith("material:") || appIcon.startsWith("svg:") || appIcon.startsWith("unicode:") || appIcon.startsWith("image:"))
                     return "";
-                return Quickshell.iconPath(appIcon, true);
+                return Paths.resolveIconPath(appIcon);
             }
 
             hasImage: hasNotificationImage
@@ -508,7 +508,7 @@ Rectangle {
                                         return appIcon;
                                     if (appIcon.startsWith("material:") || appIcon.startsWith("svg:") || appIcon.startsWith("unicode:") || appIcon.startsWith("image:"))
                                         return "";
-                                    return Quickshell.iconPath(appIcon, true);
+                                    return Paths.resolveIconPath(appIcon);
                                 }
 
                                 fallbackIcon: {