chore: add .githooks/pre-commit to auto-recompile paraglide on JSON changes

Committed hooks directory (.githooks/pre-commit) auto-runs `npm run paraglide`
and force-stages the generated JS output whenever ui/messages/*.json files are
staged, preventing svelte-check CI failures from stale paraglide output.

Added `just setup` recipe to configure core.hooksPath for new contributors.

justfile

@@ -122,6 +122,13 @@ secrets-env:
 secrets-dashboard:
     doppler open dashboard
 
+# ── Developer setup ───────────────────────────────────────────────────────────
+
+# One-time dev setup: configure git to use committed hooks in .githooks/
+setup:
+    git config core.hooksPath .githooks
+    @echo "Git hooks configured (.githooks/pre-commit active)."
+
 # ── Gitea CI ──────────────────────────────────────────────────────────────────
 
 # Validate workflow files