feat(chat,notifications): first-run chat intro card; clickable toast history + "+N" badge

- Chat: a dismissible first-run intro card (how chat works — encrypted, sent as
  shielded txs with a small fee, tied to the seed) with a "Copy my chat address"
  button, shown in the empty thread pane; dismissal persisted. The copy logic is
  shared with the existing sidebar button (moved to a helper, unchanged).
- Toasts: clicking a toast now opens the existing notification-history panel, and
  a "+N more" badge appears when several live toasts are queued (render only ever
  showed the newest). Badge opacity is a ui.toml token.

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

res/themes/ui.toml

@@ -1226,6 +1226,7 @@ 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)
+badge-fill-alpha = { size = 0.35 }    # "+N more" badge fill opacity (fraction of the accent alpha)
 glass-border-alpha = { size = 35.0 }
 progress-bar-height = { size = 2.0 }
 duration-info = { size = 1.0 }