feat: add web UI with dashboard, review queue, known objects, and settings

Vanilla HTML/CSS/JS SPA served at :5199 with dark theme:
- Dashboard: live event feed with snapshot thumbnails, filters, auto-refresh
- Review Queue: unreviewed events with tag/dismiss actions
- Known Objects: grid of tagged identities with delete
- Settings: vision backend toggle, recognition enable, retention days
- Tag modal: assign events to existing or new known objects

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

app/static/index.html

@@ -0,0 +1,61 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <title>Frigate Alerts</title>
+    <link rel="stylesheet" href="/static/style.css">
+</head>
+<body>
+    <nav id="sidebar">
+        <div class="sidebar-header">
+            <h2>Frigate Alerts</h2>
+        </div>
+        <div class="sidebar-section">
+            <a href="#" class="sidebar-item active" data-view="dashboard">Dashboard</a>
+            <a href="#" class="sidebar-item" data-view="review">Review Queue</a>
+            <a href="#" class="sidebar-item" data-view="known">Known Objects</a>
+            <a href="#" class="sidebar-item" data-view="settings">Settings</a>
+        </div>
+        <div class="sidebar-section sidebar-stats">
+            <div id="stats-summary"></div>
+        </div>
+    </nav>
+    <main id="content">
+        <div id="main-content"></div>
+    </main>
+
+    <div id="tag-modal" class="modal hidden">
+        <div class="modal-content">
+            <h3>Tag as Known Object</h3>
+            <div id="tag-modal-snapshot"></div>
+            <div class="form-group">
+                <label>Existing object:</label>
+                <select id="tag-existing-object">
+                    <option value="">-- Create new --</option>
+                </select>
+            </div>
+            <div id="tag-new-fields">
+                <div class="form-group">
+                    <label>Name:</label>
+                    <input type="text" id="tag-name" placeholder="e.g., Jeremy, Blue F-150, Max">
+                </div>
+                <div class="form-group">
+                    <label>Type:</label>
+                    <select id="tag-type">
+                        <option value="person">Person</option>
+                        <option value="vehicle">Vehicle</option>
+                        <option value="pet">Pet</option>
+                    </select>
+                </div>
+            </div>
+            <div class="modal-actions">
+                <button id="tag-save" class="btn btn-primary">Save</button>
+                <button id="tag-cancel" class="btn">Cancel</button>
+            </div>
+        </div>
+    </div>
+
+    <script src="/static/app.js"></script>
+</body>
+</html>