fix(hooks): initialize PUA_LANGUAGE to avoid unbound variable under set -u

[zhengjxu]

Problem

When ~/.pua/config.json does not exist (fresh install — user never ran /pua:survey or customized flavor), hooks/flavor-helper.sh fails with:

flavor-helper.sh: line 180: PUA_LANGUAGE: unbound variable

PUA_LANGUAGE is only assigned inside the if [ -f "$config" ] block (line 12), but referenced unconditionally at line 180 (the Chinese-language override check). Hooks running under set -u / set -eu abort on the unbound variable.

Repro

rm -f ~/.pua/config.json
bash -c 'set -eu; source hooks/flavor-helper.sh; get_flavor'
# → line 180: PUA_LANGUAGE: unbound variable

Fix

One line at the top of get_flavor():

 get_flavor() {
   local config="${HOME:-~}/.pua/config.json"
   local raw_flavor=""
+  PUA_LANGUAGE="${PUA_LANGUAGE:-}"

${PUA_LANGUAGE:-} respects any externally-set value and defaults to empty string otherwise. An empty value falls through all the Chinese-override branches correctly (they compare against "zh" / "中文").

Verification

$ rm -f ~/.pua/config.json
$ bash -c 'set -eu; source hooks/flavor-helper.sh; get_flavor; echo "flavor=$PUA_FLAVOR lang=[$PUA_LANGUAGE] OK"'
flavor=alibaba lang=[] OK

$ HOME=/tmp/nonexistent bash -c 'set -eu; source hooks/flavor-helper.sh; get_flavor; echo "no-config: flavor=$PUA_FLAVOR lang=[$PUA_LANGUAGE] OK"'
no-config: flavor=alibaba lang=[] OK

Both fresh-install (no config file) and normal paths pass under set -eu.