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/Modals/DankLauncherV2/LauncherContent.qml

@@ -789,7 +789,7 @@ FocusScope {
                 Image {
                     width: 40
                     height: 40
-                    source: editingApp?.icon ? "image://icon/" + editingApp.icon : "image://icon/application-x-executable"
+                    source: Paths.resolveIconUrl(editingApp?.icon || "application-x-executable")
                     sourceSize.width: 40
                     sourceSize.height: 40
                     fillMode: Image.PreserveAspectFit