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/Widgets/AppIconRenderer.qml

@@ -49,15 +49,7 @@ Item {
     readonly property string iconPath: {
         if (hasSpecialPrefix || !iconValue)
             return "";
-        const moddedId = Paths.moddedAppId(iconValue);
-        if (moddedId !== iconValue) {
-            if (moddedId.startsWith("~") || moddedId.startsWith("/"))
-                return Paths.toFileUrl(Paths.expandTilde(moddedId));
-            if (moddedId.startsWith("file://"))
-                return moddedId;
-            return Quickshell.iconPath(moddedId, true);
-        }
-        return Quickshell.iconPath(iconValue, true) || DesktopService.resolveIconPath(iconValue);
+        return Paths.resolveIconPath(iconValue);
     }
 
     visible: iconValue !== undefined && iconValue !== ""