feat: language switcher on portal + apply_margin re-added to Plată & total

Two fixes:

== 1. Language switcher (RO/RU/EN) in portal headers ==

New partial resources/views/partials/lang-switcher.blade.php with two
visual styles:
- style='chip' (default) — white-tinted glass for colored headers
  (shop nav bar, tracking page hero)
- style='light' — outlined buttons for pale backgrounds (invitation
  accept card)

Each button is a POST form to the existing /locale/{lang} route which:
- puts locale in session
- persists to user.locale if authenticated
- redirects back to the same page

Included in:
- resources/views/shop/layout.blade.php (right side of top nav,
  after login/register)
- resources/views/tracking/show.blade.php (top-right of hero header,
  absolutely positioned)
- resources/views/invitations/accept.blade.php (top-right of card,
  above welcome heading, light style)

Current locale button is highlighted (opaque white on colored bg /
blue on pale bg). Others are muted until hovered.

Two new translation keys:
- portal.common.language (RO: Limbă / RU: Язык)

== 2. apply_margin toggle back in "Plată & total" ==

Previous session's edit didn't persist to the file. Now confirmed in
place: WorkOrderResource form's "Plată & total" section shows a
"Aplică marjă internă" toggle between discount_pct and
override_margin_pct. Gated by FINANCE_VIEW_INTERNAL_MARGIN so only
owner / admin / manager / accountant see it. Default = true.

When toggled off on a Fișă, WorkOrderWork::saving hook writes
salary_base = total for every line on that WO (no reduction).
Backend logic already in place — this commit fixes the missing UI
control.

Suite: 303 passed (840 assertions). Unchanged — refactor + view only.

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

resources/views/shop/layout.blade.php

@@ -81,6 +81,7 @@
                 <a href="/shop/login">{{ __('portal.shop.login') }}</a>
                 <a href="/shop/register" style="background:rgba(255,255,255,.2);border-radius:6px;padding:4px 10px;">{{ __('portal.shop.register') }}</a>
             @endauth
+            @include('partials.lang-switcher')
         </nav>
     </div>
 </header>