feat(tui-go): transparent modal backdrops

Modals now composite over a dimmed copy of the screen behind them instead of an opaque
scrim, so the transcript stays faintly visible around a modal (opencode-style). center()
stashes the frame's base (View sets m.lastBase), strips+dims it to a faint scrim, and
splices the modal box over it (ANSI-aware via ansi.Truncate/TruncateLeft so the side
margins keep the dimmed content). An idempotence guard passes an already-full-screen
modal through unchanged, so the existing double-center builders don't get re-dimmed.
Geometry tests assert the composite still fills the screen exactly.

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

apps/tui-go/internal/app/model.go

@@ -311,6 +311,11 @@ type Model struct {
 	snapshotPhase bool
 	pendingEvents []protocol.ServerMessage
 
+	// lastBase is the full-screen render behind the active modal, stashed by View each
+	// frame so center() can composite a modal over a dimmed copy of it (transparent
+	// backdrop) instead of an opaque scrim. Transient render scratch — not real state.
+	lastBase string
+
 	// approval steering input buffer
 	steerBuffer string
 	steering    bool