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>

res/themes/ui.toml

@@ -1225,6 +1225,7 @@ pill-margin-y = { size = 3.0 }
 pill-rounding = { size = 12.0 }
 padding-x = { size = 12.0 }
 glass-fill-alpha = { size = 18.0 }
+accent-fill-alpha = { size = 0.22 }   # severity-tint fill opacity (fraction of the accent alpha)
 glass-border-alpha = { size = 35.0 }
 progress-bar-height = { size = 2.0 }
 duration-info = { size = 1.0 }