feat(installer): 10s reboot countdown with per-tick redraw

v0.5 used `sleep 5` after a static "System will reboot in 5 seconds."
box, which left the operator guessing how much time was left to grab
the USB stick. The new loop runs 10 → 1, clearing + redrawing the
gum-style success box each tick with the remaining-seconds figure,
giving the operator a visible window to act.

10 seconds (vs 5) because real hardware operators were missing the
window — laptops with the USB on the far side of the dock take
4-5 seconds to physically reach. 10 is comfortable, not annoying.

overlay/usr/local/bin/veilor-installer

@@ -1037,12 +1037,23 @@ run_install() {
             --show-output \
             -- bash -c 'anaconda --cmdline --kickstart=/run/install/veilor-generated.ks 2>&1 | tee /tmp/anaconda-cmdline.log' || rc=$?
         if [[ $rc -eq 0 ]]; then
-            gum style --foreground 2 --border rounded --margin "1 2" --padding "1 3" \
-                "✓ Install complete" \
-                "" \
-                "System will reboot in 5 seconds." \
-                "Remove the install media."
-            sleep 5
+            # v0.6: 10-second redraw countdown so the operator knows
+            # exactly when the reboot will fire (and has time to grab
+            # the USB stick before it boots back into the live ISO).
+            # Each tick clears + redraws the styled box with the new
+            # remaining-seconds figure — no fancy in-place repaint, just
+            # a clean re-render via clear+gum-style which the linux
+            # fbcon handles without flicker.
+            local secs
+            for secs in 10 9 8 7 6 5 4 3 2 1; do
+                clear
+                gum style --foreground 2 --border rounded --margin "1 2" --padding "1 3" \
+                    "✓ Install complete" \
+                    "" \
+                    "Rebooting in ${secs}s..." \
+                    "Remove the install media."
+                sleep 1
+            done
             systemctl reboot
         else
             prompt_error "Anaconda exited non-zero (status $rc).