Add cross-company ticket viewing and move Yritykset button to header

- tickets endpoint supports ?all=1 to fetch from all user's companies
- ticket_detail/reply/status/etc support ?company_id= for cross-company ops
- Support tab shows all companies' tickets with company badge on subject
- Yritykset button moved from tab bar to header (next to Käyttäjät)
- requireCompanyOrParam() helper for ticket endpoints

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

index.html

@@ -66,6 +66,7 @@
             <div class="header-right">
                 <span id="user-info" class="user-info"></span>
                 <button id="btn-users" class="btn-secondary" style="display:none">Käyttäjät</button>
+                <button id="btn-companies" class="btn-secondary" style="display:none">Yritykset</button>
                 <button id="btn-add" class="btn-primary">+ Lisää asiakas</button>
                 <button id="btn-logout" class="btn-secondary">Kirjaudu ulos</button>
             </div>
@@ -78,7 +79,6 @@
             <button class="tab" data-tab="leads">Liidit</button>
             <button class="tab" data-tab="archive">Arkisto</button>
             <button class="tab" data-tab="changelog">Muutosloki</button>
-            <button class="tab" data-tab="companies" id="tab-companies" style="display:none">Yritykset</button>
             <button class="tab" data-tab="settings" id="tab-settings" style="display:none">API</button>
         </div>