feat(ui): float all modals card-less like Manage Portfolio

Drop the glass card + boxed title bar from every dialog so content floats
directly on the blur backdrop under a plain h6 heading — the Manage-Portfolio
look — unifying the whole modal system on one visual language (reverses the
earlier card-for-small / float-for-large split).

- Framework: the positional BeginOverlayDialog overload (used by all ~34 card
  dialogs) now defaults to floatingContent + plainHeading (draw_helpers.h). The
  9 spec-form BlurFloat dialogs are unchanged; any dialog can still opt back to a
  card via an explicit OverlayStyle::GlassCard spec. Authored widths are kept
  (floating never resizes), so width-coupled layouts are unaffected.
- Dedup: the framework h6 title made each dialog's own repeated title heading
  redundant. Removed the duplicate body heading from Import/Export key, Backup,
  daemon update, seed backup, seed migration, Defender-blocked (app.cpp),
  Wallets, and the send-confirm popup.
- Close paths: dropping the title-bar close X is covered — every dialog has a
  footer button and/or the framework's outside-click dismiss (all pass p_open);
  send-confirm keeps Esc + Confirm/Cancel.

Verified via the headless full UI sweep on dark + light: 17 modal surfaces float
card-less with a single clean heading, nothing clipped, close controls present.
Build clean, ctest passes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app.cpp

@@ -2743,8 +2743,6 @@ void App::renderImportKeyDialog()
         return;
     }
     
-    ui::material::Type().text(ui::material::TypeStyle::H6, "Import Private Key");
-    ImGui::Dummy(ImVec2(0, ui::Layout::spacingSm()));
     ImGui::TextWrapped("Enter a private key to import. The wallet will rescan the blockchain for transactions.");
     ImGui::Spacing();
     
@@ -2847,9 +2845,7 @@ void App::renderExportKeyDialog()
         return;
     }
     
-    ui::material::Type().text(ui::material::TypeStyle::H6, "Export Private Key");
-    ImGui::Dummy(ImVec2(0, ui::Layout::spacingSm()));
-    ImGui::TextColored(ImVec4(0.9f, 0.4f, 0.4f, 1.0f), 
+    ImGui::TextColored(ImVec4(0.9f, 0.4f, 0.4f, 1.0f),
         "WARNING: Anyone with this key can spend your coins!");
     ImGui::Spacing();
     
@@ -2922,8 +2918,6 @@ void App::renderBackupDialog()
         return;
     }
     
-    ui::material::Type().text(ui::material::TypeStyle::H6, "Backup Wallet");
-    ImGui::Dummy(ImVec2(0, ui::Layout::spacingSm()));
     ImGui::TextWrapped("Export all private keys to a file. Keep this file secure!");
     ImGui::Spacing();
     
@@ -2998,8 +2992,6 @@ void App::renderDaemonUpdatePrompt()
     if (!ui::material::BeginOverlayDialog(TR("daemon_update_title"), &show_daemon_update_prompt_, 520.0f, 0.94f))
         return;
     const float dp = ui::Layout::dpiScale();
-    ui::material::Type().text(ui::material::TypeStyle::H6, TR("daemon_update_title"));
-    ImGui::Dummy(ImVec2(0, ui::Layout::spacingSm()));
     ImGui::TextWrapped("%s", TR("daemon_update_body"));
     ImGui::Spacing();
     ImGui::PushStyleColor(ImGuiCol_Text, ui::material::OnSurfaceMedium());
@@ -3071,8 +3063,6 @@ void App::renderSeedBackupDialog()
         return;
     }
 
-    ui::material::Type().text(ui::material::TypeStyle::H6, TR("seed_backup_title"));
-    ImGui::Dummy(ImVec2(0, ui::Layout::spacingSm()));
     // Button widths are logical px; BeginOverlayDialog scales the card by dpiScale() so scale these
     // to match (raw widths render small/left-packed at higher DPI / font scale).
     const float dp = ui::Layout::dpiScale();
@@ -3158,8 +3148,6 @@ void App::renderSeedMigrationDialog()
     // BeginOverlayDialog scales the card by dpiScale(); the button widths below are authored in
     // logical px and must be scaled the same way or they render small/left-packed at higher DPI.
     const float dp = ui::Layout::dpiScale();
-    ui::material::Type().text(ui::material::TypeStyle::H6, TR("mig_title"));
-    ImGui::Dummy(ImVec2(0, ui::Layout::spacingSm()));
 
     switch (seed_migration_step_) {
     case SeedMigrationStep::Intro: {
@@ -3447,9 +3435,6 @@ void App::renderAntivirusHelpDialog()
         return;
     }
     
-    ui::material::Type().text(ui::material::TypeStyle::H6, "Windows Defender Blocked xmrig");
-    ImGui::Dummy(ImVec2(0, ui::Layout::spacingSm()));
-    
     ImGui::TextWrapped(
         "Mining software is often flagged as potentially unwanted. "
         "Follow these steps to enable pool mining:");