[client] Migrate macOS cask template to Homebrew install steps

Homebrew deprecated the postflight and uninstall_preflight cask stanzas
in favour of the declarative *_steps DSL, so every brew command that
evaluates netbirdio/tap now prints deprecation warnings. Once the
deprecation becomes a disable the generated cask stops loading and
netbird-ui can no longer be installed or upgraded through Homebrew.

The *_steps blocks take JSON-serialisable steps run in a sandbox rather
than arbitrary Ruby, so system_command is re-expressed as run/remove.
The two postflight blocks merge into one because a cask carries only a
single instance, preserving the original order. set_permissions moves
from a hardcoded /Applications to base: :appdir, matching what the
installer invocation already did. The launchctl fallbacks keep their
tolerant semantics through must_succeed: false, and remove is a no-op
when the plist is absent.

client/ui/netbird-ui.rb.tmpl

@@ -17,25 +17,17 @@ cask "{{ $projectName }}" do
 
   depends_on formula: "netbird"
 
-  postflight do
-    set_permissions "/Applications/Netbird UI.app/installer.sh", '0755'
-    set_permissions "/Applications/Netbird UI.app/uninstaller.sh", '0755'
+  postflight_steps do
+    set_permissions ["Netbird UI.app/installer.sh", "Netbird UI.app/uninstaller.sh"], "0755",
+                    base: :appdir, recursive: false
+    run "Netbird UI.app/installer.sh", args: ["#{version}"], base: :appdir, sudo: true
   end
 
-  postflight do
-    system_command "#{appdir}/Netbird UI.app/installer.sh",
-                   args: ["#{version}"],
-                   sudo: true
-  end
-
-  uninstall_preflight do
-    system_command "/bin/sh",
-                   args: ["-c", <<~CMD],
-                     launchctl bootout system/netbird 2>/dev/null || \
-                       launchctl unload /Library/LaunchDaemons/netbird.plist 2>/dev/null || true
-                     rm -f /Library/LaunchDaemons/netbird.plist
-                   CMD
-                   sudo: true
+  uninstall_preflight_steps do
+    run "/bin/launchctl", args: ["bootout", "system/netbird"], sudo: true, must_succeed: false
+    run "/bin/launchctl", args: ["unload", "/Library/LaunchDaemons/netbird.plist"],
+        sudo: true, must_succeed: false
+    remove "/Library/LaunchDaemons/netbird.plist", sudo: true
   end
 
   name "Netbird UI"