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/ui/windows/wallets_dialog.h

@@ -52,8 +52,6 @@ public:
         if (s_needScan) { scan(); s_needScan = false; }
 
         if (BeginOverlayDialog(TR("wallets_title"), &s_open, 780.0f, 0.94f)) {
-            Type().text(TypeStyle::H6, TR("wallets_title"));
-            ImGui::Dummy(ImVec2(0, Layout::spacingXs()));
             Type().textColored(TypeStyle::Caption, OnSurfaceMedium(), TR("wallets_intro"));
             ImGui::Dummy(ImVec2(0, Layout::spacingSm()));