fix(security): color seed-save by outcome; explain the Encrypt button; friendly key-export errors

- Seed-backup dialog: a failed "Save to file" was drawn in the success (green)
  colour; it's now red on failure, green on success (flag reset on close).
- Encrypt-wallet dialog: the disabled Encrypt button now says why once the user
  starts typing ("Use at least 8 characters." / "The passphrases don't match."),
  and a "Show passphrase" checkbox reveals both fields.
- Key-export dialog: the raw RPC exception (e.g. "...walletpassphrase first")
  is routed through util::friendlyRpcError(), so a locked wallet reads
  "Unlock your wallet first, then try again." instead of daemon jargon.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app_security.cpp

@@ -1452,15 +1452,16 @@ void App::renderEncryptWalletDialog() {
 
                 ImGui::TextUnformatted(TR("wiz_passphrase"));
                 ImGui::PushItemWidth(-1);
-                ImGui::InputText("##enc_pass", encrypt_pass_buf_, sizeof(encrypt_pass_buf_),
-                                 ImGuiInputTextFlags_Password);
+                static bool s_showEncPass = false;
+                const ImGuiInputTextFlags encPwFlags = s_showEncPass ? 0 : ImGuiInputTextFlags_Password;
+                ImGui::InputText("##enc_pass", encrypt_pass_buf_, sizeof(encrypt_pass_buf_), encPwFlags);
                 ImGui::PopItemWidth();
 
                 ImGui::TextUnformatted(TR("enc_confirm"));
                 ImGui::PushItemWidth(-1);
-                ImGui::InputText("##enc_confirm", encrypt_confirm_buf_, sizeof(encrypt_confirm_buf_),
-                                 ImGuiInputTextFlags_Password);
+                ImGui::InputText("##enc_confirm", encrypt_confirm_buf_, sizeof(encrypt_confirm_buf_), encPwFlags);
                 ImGui::PopItemWidth();
+                ImGui::Checkbox(TR("enc_show_pass"), &s_showEncPass);
 
                 // Strength meter bar
                 {
@@ -1508,8 +1509,15 @@ void App::renderEncryptWalletDialog() {
                 }
 
                 ImGui::Spacing();
-                bool valid = strlen(encrypt_pass_buf_) >= 8 &&
-                             strcmp(encrypt_pass_buf_, encrypt_confirm_buf_) == 0;
+                const size_t encPassLen = strlen(encrypt_pass_buf_);
+                const bool encMatch = strcmp(encrypt_pass_buf_, encrypt_confirm_buf_) == 0;
+                bool valid = encPassLen >= 8 && encMatch;
+                // Say WHY the button is disabled, once the user has started typing (was silent before).
+                if (!valid && (encPassLen > 0 || strlen(encrypt_confirm_buf_) > 0)) {
+                    const char* why = (encPassLen < 8) ? TR("enc_min_len")
+                                    : (!encMatch ? TR("enc_mismatch") : "");
+                    if (*why) ImGui::TextColored(ImVec4(1.0f, 0.7f, 0.3f, 1.0f), "%s", why);
+                }
 
                 float btnW = (ImGui::GetContentRegionAvail().x - ImGui::GetStyle().ItemSpacing.x) * 0.5f;
                 ImGui::BeginDisabled(!valid || encrypt_in_progress_);