Add multi-tenant support with per-company data isolation

Implement full multi-company architecture:
- Per-company directory structure (data/companies/{id}/)
- Automatic migration from single-tenant to multi-tenant
- Company management admin tab (create, edit, delete companies)
- Per-company IMAP mailbox configuration (multiple mailboxes per company)
- User access control per company (companies array on users)
- Company switcher in header (shown when user has access to >1 company)
- Session-based company context with check_auth fallback for old sessions
- Ticket list shows mailbox name instead of sender
- IMAP settings moved from global config to company-specific config
- All data endpoints protected with requireCompany() guard

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

style.css

@@ -1321,3 +1321,43 @@ span.empty {
     color: #999;
     font-size: 0.85rem;
 }
+
+/* Company selector */
+.company-selector {
+    margin-left: 1.5rem;
+    padding: 6px 12px;
+    border: 2px solid #e0e0e0;
+    border-radius: 8px;
+    font-size: 0.88rem;
+    font-weight: 600;
+    color: #0f3460;
+    background: #f8f9fb;
+    cursor: pointer;
+    transition: border-color 0.2s;
+}
+
+.company-selector:hover,
+.company-selector:focus {
+    border-color: #0f3460;
+    outline: none;
+}
+
+/* Mailbox items */
+.mailbox-item {
+    transition: border-color 0.2s;
+}
+
+.mailbox-item:hover {
+    border-color: #0f3460 !important;
+}
+
+/* Company badge */
+.company-badge {
+    display: inline-block;
+    padding: 2px 8px;
+    border-radius: 4px;
+    font-size: 0.75rem;
+    font-weight: 600;
+    background: #e8f0fe;
+    color: #1a56db;
+}