feat: add Telegram Desktop package

Repackages the official static tsetup.7.0.5.tar.xz Linux build for
amd64, keeping the upstream org.telegram.desktop desktop-id and
TelegramDesktop WM class.

telegram-desktop/postinstall.sh

@@ -0,0 +1,23 @@
+#!/bin/bash
+set -euo pipefail
+
+optional_refresh() {
+    local label=$1
+    shift
+    if ! "$@"; then
+        echo "Предупреждение: не удалось обновить ${label}." >&2
+    fi
+}
+
+if command -v update-desktop-database >/dev/null 2>&1; then
+    optional_refresh "desktop-базу" update-desktop-database -q /usr/share/applications
+fi
+if command -v gtk-update-icon-cache >/dev/null 2>&1; then
+    optional_refresh "кэш иконок" \
+        gtk-update-icon-cache -q -t -f /usr/share/icons/hicolor
+fi
+if command -v kbuildsycoca6 >/dev/null 2>&1; then
+    optional_refresh "кэш KDE" kbuildsycoca6 --noincremental
+elif command -v kbuildsycoca5 >/dev/null 2>&1; then
+    optional_refresh "кэш KDE" kbuildsycoca5 --noincremental
+fi