fix(tui): center dialog properly and prevent overflow

Use justifyContent="center" instead of fixed paddingTop for true
vertical centering. Add maxHeight to prevent dialog from overflowing
when terminal is small.

Fixes anomalyco#7902

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: zerone0x

packages/opencode/src/cli/cmd/tui/ui/dialog.tsx

@@ -25,8 +25,8 @@ export function Dialog(
       width={dimensions().width}
       height={dimensions().height}
       alignItems="center"
+      justifyContent="center"
       position="absolute"
-      paddingTop={dimensions().height / 4}
       left={0}
       top={0}
       backgroundColor={RGBA.fromInts(0, 0, 0, 150)}
@@ -38,6 +38,7 @@ export function Dialog(
         }}
         width={props.size === "large" ? 80 : 60}
         maxWidth={dimensions().width - 2}
+        maxHeight={dimensions().height - 4}
         backgroundColor={theme.backgroundPanel}
         paddingTop={1}
       >