Center the timezone picker in first-boot setup

The timezone prompt falls back to `gum filter` when tzupdate can't guess a
zone, which is the common case on a first boot with no network yet. That was
the one widget in the form without a GUM_*_PADDING export, so it drew flush
left while every other step sat centered under the logo.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

bin/omarchy-provision-owner

@@ -114,6 +114,7 @@ measure_terminal() {
 
   local padding="0 0 0 $PADDING_LEFT"
   export GUM_CHOOSE_PADDING="$padding"
+  export GUM_FILTER_PADDING="$padding"
   export GUM_INPUT_PADDING="$padding"
   export GUM_SPIN_PADDING="$padding"
   export GUM_TABLE_PADDING="$padding"