Integrate web_template design system and fix security/quality issues

Security fixes:
- Add HTTP method validation to delete_comment.php (block CSRF via GET)
- Remove $_GET fallback in comment deletion (was CSRF bypass vector)
- Guard session_start() with session_status() check across API files
- Escape json_encode() data attributes with htmlspecialchars in views
- Escape inline APP_TIMEZONE config values in DashboardView/TicketView
- Validate timezone param against DateTimeZone::listIdentifiers() in index.php
- Remove Database::escape() (was using real_escape_string, not safe)
- Fix AttachmentModel hardcoded connection; inject via constructor

Backend fixes:
- Fix CommentModel bind_param type for ticket_id (s→i)
- Fix buildCommentThread orphan parent guard
- Fix StatsModel JOIN→LEFT JOIN so unassigned tickets aren't excluded
- Add ticket ID validation in BulkOperationsModel before implode()
- Add duplicate key retry in TicketModel::createTicket() for race conditions
- Wrap SavedFiltersModel default filter changes in transactions
- Add null result guards in WorkflowModel query methods

Frontend JS:
- Rewrite toast.js as lt.toast shim (base.js dependency)
- Delegate escapeHtml() to lt.escHtml()
- Rewrite keyboard-shortcuts.js using lt.keys.on()
- Migrate settings.js to lt.api.* and lt.modal.open/close()
- Migrate advanced-search.js to lt.api.* and lt.modal.open/close()
- Migrate dashboard.js fetch calls to lt.api.*; update all dynamic
  modals (bulk ops, quick actions, confirm/input) to lt-modal structure
- Migrate ticket.js fetchMentionUsers to lt.api.get()
- Remove console.log/error/warn calls from JS files

Views:
- Add /web_template/base.css and base.js to all 10 view files
- Call lt.keys.initDefaults() in DashboardView, TicketView, admin views
- Migrate all modal HTML from settings-modal/settings-content to
  lt-modal-overlay/lt-modal/lt-modal-header/lt-modal-body/lt-modal-footer
- Replace style="display:none" with aria-hidden="true" on all modals
- Replace modal open/close style.display with lt.modal.open/close()
- Update modal buttons to lt-btn lt-btn-primary/lt-btn-ghost classes
- Remove manual ESC keydown handlers (replaced by lt.keys.initDefaults)
- Fix unescaped timezone values in TicketView inline script

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

views/CreateTicketView.php

@@ -11,8 +11,10 @@ $nonce = SecurityHeadersMiddleware::getNonce();
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>Create New Ticket</title>
     <link rel="icon" type="image/png" href="<?php echo $GLOBALS['config']['ASSETS_URL']; ?>/images/favicon.png">
+    <link rel="stylesheet" href="/web_template/base.css">
     <link rel="stylesheet" href="<?php echo $GLOBALS['config']['ASSETS_URL']; ?>/css/dashboard.css?v=20260126c">
     <link rel="stylesheet" href="<?php echo $GLOBALS['config']['ASSETS_URL']; ?>/css/ticket.css?v=20260124e">
+    <script nonce="<?php echo $nonce; ?>" src="/web_template/base.js"></script>
     <script nonce="<?php echo $nonce; ?>" src="<?php echo $GLOBALS['config']['ASSETS_URL']; ?>/js/utils.js"></script>
     <script nonce="<?php echo $nonce; ?>" src="<?php echo $GLOBALS['config']['ASSETS_URL']; ?>/js/dashboard.js?v=20260205"></script>
     <script nonce="<?php echo $nonce; ?>">
@@ -61,7 +63,7 @@ $nonce = SecurityHeadersMiddleware::getNonce();
         <div class="ascii-content">
             <div class="ascii-frame-inner">
                 <div class="error-message" style="color: var(--priority-1); border: 2px solid var(--priority-1); padding: 1rem; background: rgba(231, 76, 60, 0.1);">
-                    <strong>⚠ Error:</strong> <?php echo $error; ?>
+                    <strong>⚠ Error:</strong> <?php echo htmlspecialchars($error, ENT_QUOTES, 'UTF-8'); ?>
                 </div>
             </div>
         </div>