Adds self-hosted update system

Implements a self-hosted update mechanism, allowing the application to be updated from a corporate network share without relying on third-party services.

This includes:
- Functionality to check for updates
- Download installers
- Verify checksums
- Install updates with UAC elevation

Configuration is managed via the config.ini file, with automatic checks on startup. A new settings UI is also included.

installer/installer.iss

@@ -5,13 +5,18 @@
 [Setup]
 AppName={#ApplicationName}
 AppVersion={#ApplicationVersion}
+; Default directory (will be adjusted in code based on installation mode)
+; Admin mode: C:\Program Files\EMLy
+; User mode:  C:\Users\{username}\AppData\Local\Programs\EMLy
 DefaultDirName={autopf}\EMLy
 OutputBaseFilename={#ApplicationName}_Installer_{#ApplicationVersion}
 ArchitecturesInstallIn64BitMode=x64compatible
 DisableProgramGroupPage=yes
-; Request administrative privileges for HKA to write to HKLM if needed, 
-; or use "lowest" if purely per-user, but file associations usually work better with admin rights or proper HKA handling.
-PrivilegesRequired=admin
+; Allow user to choose between admin (system-wide) and user-only install
+; "lowest" = does not require admin privileges by default (user mode)
+; "dialog" = shows a dialog asking user to choose installation mode
+PrivilegesRequired=lowest
+PrivilegesRequiredOverridesAllowed=dialog
 SetupIconFile=..\build\windows\icon.ico
 UninstallDisplayIcon={app}\{#ApplicationName}.exe
 AppVerName={#ApplicationName} {#ApplicationVersion}
@@ -22,6 +27,12 @@ Source: "..\build\bin\{#ApplicationName}.exe"; DestDir: "{app}"; Flags: ignoreve
 Source: "..\build\bin\config.ini"; DestDir: "{app}"; Flags: ignoreversion
 
 [Registry]
+; File associations using HKA (HKEY_AUTO) registry root
+; HKA automatically selects the appropriate registry hive:
+; - HKLM (HKEY_LOCAL_MACHINE) for admin/system-wide installations
+; - HKCU (HKEY_CURRENT_USER) for user-only installations
+; This ensures file associations work correctly for both installation modes
+
 ; 1. Register the .eml extension and point it to our internal ProgID "EMLy.EML"
 Root: HKA; Subkey: "Software\Classes\.eml"; ValueType: string; ValueName: ""; ValueData: "{#ApplicationName}.EML"; Flags: uninsdeletevalue
 Root: HKA; Subkey: "Software\Classes\.msg"; ValueType: string; ValueName: ""; ValueData: "{#ApplicationName}.MSG"; Flags: uninsdeletevalue
@@ -44,3 +55,26 @@ Root: HKA; Subkey: "Software\Classes\{#ApplicationName}.MSG\shell\open"; ValueTy
 
 [Icons]
 Name: "{autoprograms}\{#ApplicationName}"; Filename: "{app}\{#ApplicationName}.exe"
+
+[Code]
+// Override default directory based on installation mode
+function GetDefaultDirName(Param: string): string;
+begin
+  // If installing with admin privileges (system-wide), use Program Files
+  if IsAdminInstallMode then
+    Result := ExpandConstant('{autopf}\{#ApplicationName}')
+  // If installing for current user only, use AppData\Local\Programs
+  else
+    Result := ExpandConstant('{localappdata}\Programs\{#ApplicationName}');
+end;
+
+procedure CurPageChanged(CurPageID: Integer);
+begin
+  // Update the directory when the directory selection page is shown
+  if CurPageID = wpSelectDir then
+  begin
+    // Only set if user hasn't manually changed it
+    if WizardForm.DirEdit.Text = ExpandConstant('{autopf}\{#ApplicationName}') then
+      WizardForm.DirEdit.Text := GetDefaultDirName('');
+  end;
+end;