feat: gradient buttons, language selector with flags, SVG support, and UI improvements

- Add linear gradient backgrounds to primary/secondary/danger buttons
- Add language selector with flag icons in ComboBox
- Add SVG support via egui_extras feature
- Add cancel confirmation dialog on exit
- Update Yali logo to SVG icon
- Add flag SVGs for all countries
- Improve users step layout with form helpers
- Refactor button API (ui::theme::*_button now takes ui param)

locales/en.toml

@@ -4,6 +4,10 @@ back         = "Back"
 next         = "Next"
 cancel       = "Cancel"
 confirm      = "Confirm"
+cancel_confirm_title  = "Cancel Installation"
+cancel_confirm_msg    = "The installation will be cancelled. Are you sure?"
+cancel_confirm_yes    = "Yes, Cancel"
+cancel_confirm_no     = "No, Continue"
 
 # ── Welcome ───────────────────────────────────────────────
 welcome             = "Welcome"
@@ -53,7 +57,7 @@ users_title             = "User Account"
 users_description       = "Create a user account for your system."
 real_name_label         = "Full Name"
 username_label          = "Username"
-password_label          = "Password"
+password_label          = "Password (at least 8 characters...)"
 password_confirm_label  = "Confirm password"
 hostname_label          = "Computer name"
 username_invalid        = "Invalid username. Must start with a lowercase letter or underscore, max 32 characters."