Escape .desktop values with parameter expansion, not sed

GNU sed's N auto-prints the pattern space and exits at end of input, so the
`:a;N;$!ba` slurp skipped every following s/// for a value with no newline in
it. That is every value except the injection attempt the escaping exists to
stop, so the Exec quoting's own backslashes were written to the file unescaped
and GLib refused to load the entry.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-Authored-By: Codex GPT-5.6-sol (xhigh) <noreply@openai.com>

bin/omarchy-webapp-install

@@ -70,9 +70,20 @@ desktop_string_escape() {
   # a raw newline would start a new key line and let a value inject a second
   # Exec=. Escape backslash first, then tab/CR/LF and a leading space. Every value
   # written into the .desktop file passes through here.
-  printf '%s' "$1" \
-    | sed -e ':a;N;$!ba' \
-          -e 's/\\/\\\\/g' -e 's/\t/\\t/g' -e 's/\r/\\r/g' -e 's/\n/\\n/g' -e 's/^ /\\s/'
+  #
+  # Parameter expansion rather than sed: GNU sed's N auto-prints the pattern space
+  # and exits at end of input, so a `:a;N;$!ba` slurp skips every following s///
+  # for a value with no newline in it - which is every value except the injection
+  # attempt this exists to stop.
+  local value="$1"
+
+  value=${value//\\/\\\\}
+  value=${value//$'\t'/\\t}
+  value=${value//$'\r'/\\r}
+  value=${value//$'\n'/\\n}
+  [[ $value == " "* ]] && value="\\s${value# }"
+
+  printf '%s' "$value"
 }
 
 desktop_exec_arg() {