feat(tools): add Brave tab and Umbriel window management helpers

dot_local/bin/executable_umbriel-float-square

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+#!/bin/sh
+# umbriel-float-square
+# Floating focused window: square, as tall as the usable height (below
+# Noctalia's bar) minus PAD px above and below, centered on its output.
+# Tiled: center its column (the old Mod+C). Extents are fractions of the
+# usable area, so both sides are computed from the output's usable size.
+set -eu
+
+PAD=4
+
+window=$(umbriel windows --json | jq -c '[.[] | select(.active)][0] // empty')
+[ -n "$window" ] || exit 0
+
+if [ "$(printf '%s' "$window" | jq -r .floating)" != true ]; then
+    exec umbriel msg column-center
+fi
+
+id=$(printf '%s' "$window" | jq -r .id)
+output=$(printf '%s' "$window" | jq -r .output)
+extents=$(umbriel outputs --json | jq -r --arg o "$output" --argjson pad "$PAD" '
+    def frac: . * 10000 | floor / 10000;
+    .[] | select(.name == $o) | .usable_area
+    | (.height - 2 * $pad) as $side
+    | "\($side) \([$side / .width, 1] | min | frac) \($side / .height | frac)"')
+set -- $extents
+side=$1
+
+umbriel msg "window-set-secondary-extent:$3"
+umbriel msg "window-set-primary-extent:$2"
+
+# window-center uses the size the client has committed; centering before the
+# resize lands centers the old width. Wait up to 1 s for the new width.
+i=0
+while [ $i -lt 20 ]; do
+    w=$(umbriel windows --json | jq -r --arg id "$id" '.[] | select(.id == $id) | .w')
+    [ $((w - side)) -ge -1 ] && [ $((w - side)) -le 1 ] && break
+    sleep 0.05
+    i=$((i + 1))
+done
+exec umbriel msg window-center