[release] Write fresh-install breadcrumb from installers

Installers write a .fresh-install breadcrumb on fresh installs only and
delete stale breadcrumbs on upgrade; none of them writes login items or
registry Run keys. Windows NSIS detects upgrades via the uninstall
registry entry or an existing installed executable; the macOS pkg via the
previous pkgutil receipt; Linux deb/rpm via the standard postinstall
arguments. Post-update GUI relaunches (macOS open, Linux
ui-post-install.sh) pass --post-update so the first-run autostart default
cannot fire on updates.

client/ui/build/windows/nsis/project.nsi

@@ -75,6 +75,8 @@ OutFile "..\..\..\bin\${INFO_PROJECTNAME}-${ARCH}-installer.exe" # Name of the i
 InstallDir "$PROGRAMFILES64\${INFO_COMPANYNAME}\${INFO_PRODUCTNAME}" # Default installing folder ($PROGRAMFILES is Program Files folder).
 ShowInstDetails show # This will always show the installation details.
 
+Var FreshInstall
+
 Function .onInit
    !insertmacro wails.checkArchitecture
 FunctionEnd
@@ -84,16 +86,38 @@ Section
 
     !insertmacro wails.webview2runtime
 
+    # Fresh-vs-upgrade detection must run before any file is written. An
+    # existing uninstall registry entry or an already-installed executable
+    # means upgrade; only a true fresh install gets the breadcrumb the GUI
+    # uses to enable launch-on-login by default. The installer itself never
+    # writes login items or Run keys.
+    StrCpy $FreshInstall "1"
+    ReadRegStr $0 HKLM "${UNINST_KEY}" "UninstallString"
+    StrCmp $0 "" +2 0
+        StrCpy $FreshInstall "0"
+    IfFileExists "$INSTDIR\${PRODUCT_EXECUTABLE}" 0 +2
+        StrCpy $FreshInstall "0"
+
     SetOutPath $INSTDIR
-    
+
     !insertmacro wails.files
 
+    # On upgrade, delete any stale breadcrumb (covers a fresh install where
+    # the GUI never launched before the first update).
+    StrCmp $FreshInstall "1" 0 removeBreadcrumb
+        FileOpen $0 "$INSTDIR\.fresh-install" w
+        FileClose $0
+        Goto breadcrumbDone
+    removeBreadcrumb:
+        Delete "$INSTDIR\.fresh-install"
+    breadcrumbDone:
+
     CreateShortcut "$SMPROGRAMS\${INFO_PRODUCTNAME}.lnk" "$INSTDIR\${PRODUCT_EXECUTABLE}"
     CreateShortCut "$DESKTOP\${INFO_PRODUCTNAME}.lnk" "$INSTDIR\${PRODUCT_EXECUTABLE}"
 
     !insertmacro wails.associateFiles
     !insertmacro wails.associateCustomProtocols
-    
+
     !insertmacro wails.writeUninstaller
 SectionEnd