feat: Add chat filter functionality for channel and DM messages

- Add filter-utils.js with diacritic-insensitive search and text highlighting
- Add FAB filter button (gray funnel icon) to channel chat and DM
- Filter bar slides in as overlay at top of chat area
- Real-time filtering with debounce (150ms) as user types
- Matched text highlighted with yellow background
- Support for Polish characters (wol matches wół)
- Keyboard shortcuts: Ctrl+F to open, Escape to close
- Match counter shows X / Y filtered messages
- Filter persists during auto-refresh

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/static/css/style.css

@@ -1067,3 +1067,141 @@ main {
     color: #75b798;
     background-color: rgba(117, 183, 152, 0.15);
 }
+
+/* =============================================================================
+   Chat Filter
+   ============================================================================= */
+
+/* Filter FAB button (gray gradient) */
+.fab-filter {
+    background: linear-gradient(135deg, #6c757d 0%, #495057 100%);
+    color: white;
+}
+
+/* Filter bar overlay - slides down from top of chat area */
+.filter-bar {
+    position: absolute;
+    top: 0;
+    left: 0;
+    right: 0;
+    z-index: 1002;  /* Above mentions popup (1001) */
+    background-color: #ffffff;
+    border-bottom: 1px solid #dee2e6;
+    box-shadow: 0 2px 8px rgba(0, 0, 0, 0.15);
+    padding: 0.75rem;
+    transform: translateY(-100%);
+    opacity: 0;
+    visibility: hidden;
+    transition: transform 0.3s ease, opacity 0.3s ease, visibility 0.3s ease;
+}
+
+.filter-bar.visible {
+    transform: translateY(0);
+    opacity: 1;
+    visibility: visible;
+}
+
+/* Filter bar inner layout */
+.filter-bar-inner {
+    display: flex;
+    align-items: center;
+    gap: 0.5rem;
+}
+
+.filter-bar-input {
+    flex: 1;
+    border-radius: 0.375rem;
+    border: 1px solid #ced4da;
+    padding: 0.5rem 0.75rem;
+    font-size: 0.9rem;
+}
+
+.filter-bar-input:focus {
+    outline: none;
+    border-color: #86b7fe;
+    box-shadow: 0 0 0 0.2rem rgba(13, 110, 253, 0.25);
+}
+
+/* Filter bar buttons */
+.filter-bar-btn {
+    width: 36px;
+    height: 36px;
+    border-radius: 50%;
+    border: none;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    font-size: 1rem;
+    cursor: pointer;
+    transition: background-color 0.15s ease;
+}
+
+.filter-bar-btn-clear {
+    background-color: #f8f9fa;
+    color: #6c757d;
+}
+
+.filter-bar-btn-clear:hover {
+    background-color: #e9ecef;
+}
+
+.filter-bar-btn-close {
+    background-color: #dc3545;
+    color: white;
+}
+
+.filter-bar-btn-close:hover {
+    background-color: #bb2d3b;
+}
+
+/* Filter match count indicator */
+.filter-match-count {
+    font-size: 0.75rem;
+    color: #6c757d;
+    white-space: nowrap;
+    padding: 0 0.5rem;
+}
+
+/* Highlighted text in filtered messages */
+.filter-highlight {
+    background-color: #fff3cd;
+    border-radius: 0.2rem;
+    padding: 0 0.1rem;
+}
+
+/* Hidden messages when filtering */
+.message-wrapper.filter-hidden,
+.dm-message.filter-hidden {
+    display: none !important;
+}
+
+/* No matches message */
+.filter-no-matches {
+    text-align: center;
+    padding: 2rem;
+    color: #6c757d;
+}
+
+.filter-no-matches i {
+    font-size: 2rem;
+    margin-bottom: 0.5rem;
+    display: block;
+}
+
+/* Mobile responsive filter bar */
+@media (max-width: 576px) {
+    .filter-bar {
+        padding: 0.5rem;
+    }
+
+    .filter-bar-input {
+        font-size: 0.85rem;
+        padding: 0.4rem 0.6rem;
+    }
+
+    .filter-bar-btn {
+        width: 32px;
+        height: 32px;
+        font-size: 0.9rem;
+    }
+}