fix(admin): two more apostrophe-in-translation regressions + UX rework

UX rework — recipe assignment modal on /admin/stations
- Replace the single "select recipe + Assegna" dropdown with a two-
  column layout: Ricette disponibili (left) and Assegnate alla
  stazione (right), each row with an inline action button. Top search
  filter narrows both columns at once. Empty states explain *why* the
  list is empty (no recipes in system, all already assigned, no match
  for the filter).
- Rationale: the old dropdown silently hid every option once a recipe
  was assigned, leaving the user unable to tell whether the system
  was broken or simply out of unassigned recipes.

Apostrophe regressions
- /admin/stations alert/errorMsg literals reworded with double-quoted
  outer JS strings ("Errore nella eliminazione" / "...assegnazione").
- /admin/users toggle confirm modal: x-text expression contained
  '{{ _('… l\'utente') }}'. Inside a Jinja-rendered HTML attribute,
  the apostrophe in "l'utente" closed the JS literal early, killing
  the binding. Fixed by using " as the JS string delimiter so
  the inner apostrophe is harmless.

Alpine x-if templates can't host nested templates
- Replaced two nested-template empty-state blocks with x-text bound
  to computed getters (unassignedEmptyMessage,
  assignedEmptyMessage). Alpine errored with
  "Cannot set properties of null (setting '_x_dataStack')" when the
  outer template's child wasn't a single root element.

Test guard widened
- src/frontend/flask_app/tests/test_template_js_syntax.py now also
  parses every Alpine attribute (x-*, @*, :*) on the rendered HTML
  and runs `node --check` on each expression wrapped in `void (…)`.
  Previously it only inspected inline <script> bodies, which is why
  the x-text bug on /admin/users slipped through. Verified the
  extended test catches the original l'utente regression by reverting
  + running + restoring.

Layout regression — UPLOAD_DIR defaulted to server/uploads
- The previous .env.example shipped UPLOAD_DIR=server/uploads, which
  matched the V1.x layout but pointed outside the new project tree.
  Updated to UPLOAD_DIR=uploads so files land in the project-root
  uploads/ volume that src/backend/config.py.upload_path resolves.
- Added uploads/general/ to .gitignore (per-user uploads, not source).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.env.example

@@ -26,7 +26,10 @@ API_SERVER_URL=http://localhost:8000
 STATION_CODE=ST-DEFAULT
 
 # --- File Storage ---
-UPLOAD_DIR=server/uploads
+# Resolved against the project root in src/backend/config.py.
+# Default "uploads" maps to <project_root>/uploads, mounted as a Docker
+# volume in production.
+UPLOAD_DIR=uploads
 MAX_UPLOAD_SIZE_MB=50
 
 # --- Setup Page ---