Fix silent launch failure for apps whose desktop id ends in .desktop (Telegram) (#6418)

* Launch apps by their full desktop file name

gtk-launch only appends the .desktop suffix when the argument doesn't
already end with it. Apps whose desktop id itself ends in .desktop --
notably Telegram (org.telegram.desktop.desktop) -- were therefore looked
up as a nonexistent file and silently failed to launch from the menu,
while every other app worked.

Passing the full file name to gtk-launch resolves the entry
unambiguously for both naming styles.

* Clarify launch comment wording

test/shell.d/app-search-test.sh

@@ -104,6 +104,11 @@ assert(
   'app library runs desktop entry launch through the shell'
 )
 
+assert(
+  appLibraryQml.includes('Util.shellQuote(id + ".desktop")'),
+  'app library launches by full file name so ids ending in .desktop (org.telegram.desktop) resolve'
+)
+
 assert(
   /function iconIndexScanCommand\(\)[\s\S]*-path "\*\/apps\/\*" -o -path "\*\/devices\/\*"/.test(appLibraryQml),
   'app library fallback icon index includes device icons'