feat(icons): support file path substitutions in getAppIcon (#1867)

Allow appIdSubstitutions to return absolute file paths (/, ~, file://)
that bypass Quickshell.iconPath theme lookup. This enables users to map
app IDs directly to icon files on disk via the existing substitution UI.

Fixes PWA icon resolution for Chrome, Chromium and Edge PWAs where
Qt's icon theme lookup fails to find icons installed to
~/.local/share/icons/hicolor/ by the browser.

Example substitutions (Settings → Running Apps → App ID Substitutions):

  ^msedge-_(.+)$ → ~/.local/share/icons/hicolor/128x128/apps/msedge-$1.png
  ^(chrome|msedge|chromium)-(.+)$ → ~/.local/share/icons/hicolor/128x128/apps/$1-$2.png

Tested with Chrome PWAs (YouTube, Twitch, ai-ta) and Edge PWAs
(Microsoft Teams, Outlook) on niri/Wayland.

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

quickshell/Common/Paths.qml

@@ -78,6 +78,10 @@ Singleton {
 
         const moddedId = moddedAppId(appId);
         if (moddedId !== appId) {
+            if (moddedId.startsWith("~") || moddedId.startsWith("/"))
+                return toFileUrl(expandTilde(moddedId));
+            if (moddedId.startsWith("file://"))
+                return moddedId;
             return Quickshell.iconPath(moddedId, true);
         }