feat(balance,notifications): shielded-card tooltip + clearer toast severity

- The Shielded balance card gains the explanatory hover tooltip the mining card
  already has (suppressed while the pending-chip tooltip is showing).
- Toasts get a severity-tinted background fill and a slightly thicker accent
  border, so Success/Warning/Error/Info differ by more than hue alone (the
  per-severity leading icon already existed). Tint opacity is a ui.toml token.

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

src/ui/notifications.cpp

@@ -123,9 +123,16 @@ void Notifications::render()
     glassSpec.borderWidth = 1.0f;
     material::DrawGlassPanel(dl, pMin, pMax, glassSpec);
 
+    // Severity-tinted fill so the toast differs by more than hue (helps when color alone is hard to read).
+    {
+        ImVec4 tint = accent_color;   // already premultiplied by fade `alpha`
+        tint.w *= nde("accent-fill-alpha", 0.22f);
+        dl->AddRectFilled(pMin, pMax, ImGui::ColorConvertFloat4ToU32(tint), pillRounding);
+    }
+
     // Colored accent border on top of the glass border for type indication
     ImU32 accentCol = ImGui::ColorConvertFloat4ToU32(accent_color);
-    dl->AddRect(pMin, pMax, accentCol, pillRounding, 0, 1.0f);
+    dl->AddRect(pMin, pMax, accentCol, pillRounding, 0, 1.5f * dp);
 
     // Progress bar at bottom of pill (accent-colored), clipped to pill rounded
     // corners.  Draw a full-pill-size rounded rect and clip it to just the