Copy 24-word seed phrase via auto-clearing clipboard helper

Both seed-copy buttons (wallet creation and onboarding) now use
copy_secret_to_buffer, which compare-then-clears the clipboard after a
delay, matching the nsec copy path, instead of the non-clearing
copy_string_to_buffer.

src/gui/views/goblin/onboarding.rs

@@ -666,7 +666,9 @@ impl OnboardingContent {
 				t!("goblin.onboarding.words.copy_clipboard").to_string()
 			};
 			if w::chip(ui, &label, false).clicked() {
-				cb.copy_string_to_buffer(self.mnemonic_setup.mnemonic.get_phrase());
+				// Secret: auto-clears from the clipboard after a delay
+				// (compare-then-clear) so the seed phrase does not linger there.
+				cb.copy_secret_to_buffer(self.mnemonic_setup.mnemonic.get_phrase());
 				cb.vibrate_copy();
 				self.words_copied = Some(std::time::Instant::now());
 			}

src/gui/views/wallets/creation/content.rs

@@ -231,7 +231,10 @@ impl WalletCreationContent {
 							PhraseMode::Generate => {
 								let c_t = format!("{} {}", COPY, t!("copy"));
 								View::button(ui, c_t, Colors::white_or_black(false), || {
-									cb.copy_string_to_buffer(
+									// Secret: auto-clears from the clipboard after a
+									// delay (compare-then-clear) so the seed phrase
+									// does not linger there, matching the nsec.
+									cb.copy_secret_to_buffer(
 										self.mnemonic_setup.mnemonic.get_phrase(),
 									);
 								});