[ci] Fix Windows installer icon/banner paths missed in ui-wails rename

The ui-wails -> ui rename deleted the fyne installer assets but left the
NSIS and WiX scripts pointing at client/ui/assets/netbird.ico, which broke
the Windows Installer CI job. Point both scripts at the Wails-side icon
(client/ui/build/windows/icon.ico) and restore banner.bmp into the new
build directory so the NSIS welcome/finish sidebar keeps rendering.

client/installer.nsis

@@ -6,7 +6,7 @@
 !define DESCRIPTION "Connect your devices into a secure WireGuard-based overlay network with SSO, MFA, and granular access controls."
 !define INSTALLER_NAME "netbird-installer.exe"
 !define MAIN_APP_EXE "Netbird"
-!define ICON "ui\\assets\\netbird.ico"
+!define ICON "ui\\build\\windows\\icon.ico"
 !define BANNER "ui\\build\\banner.bmp"
 !define LICENSE_DATA "..\\LICENSE"
 

client/netbird.wxs

@@ -123,7 +123,7 @@
 		</InstallExecuteSequence>
 
 		<!-- Icons -->
-		<Icon Id="NetbirdIcon" SourceFile=".\client\ui\assets\netbird.ico" />
+		<Icon Id="NetbirdIcon" SourceFile=".\client\ui\build\windows\icon.ico" />
 		<Property Id="ARPPRODUCTICON" Value="NetbirdIcon" />
 
 	</Package>