feat(ui): resizable rail and peek pane

- Drag handles between rail/center and center/peek
- Rail: 120–360px range, peek: 250–700px range
- Widths persisted in localStorage
- Handles hidden when panel is collapsed (zen mode)
- Transition disabled during drag for smooth resize

web/style.css

@@ -177,17 +177,34 @@ nav { display: flex; gap: 2px; }
 /* ── MAIN LAYOUT ────────────────────────────────────── */
 main {
     display: grid;
-    grid-template-columns: 192px 1fr 400px;
+    grid-template-columns: var(--rail-w, 192px) 4px 1fr 4px var(--peek-w, 400px);
     overflow: hidden;
-    transition: grid-template-columns var(--t-base);
 }
 
-main.hide-rail { grid-template-columns: 0px 1fr 400px; }
-main.hide-peek { grid-template-columns: 192px 1fr 0px; }
-main.hide-rail.hide-peek { grid-template-columns: 0px 1fr 0px; }
+main.resizing { transition: none; }
+main:not(.resizing) { transition: grid-template-columns var(--t-base); }
 
-main.hide-rail #tag-rail { overflow: hidden; border-right: none; }
-main.hide-peek #detail-pane { overflow: hidden; border-left: none; }
+main.hide-rail { grid-template-columns: 0px 0px 1fr 4px var(--peek-w, 400px); }
+main.hide-peek { grid-template-columns: var(--rail-w, 192px) 4px 1fr 0px 0px; }
+main.hide-rail.hide-peek { grid-template-columns: 0px 0px 1fr 0px 0px; }
+
+main.hide-rail #tag-rail { overflow: hidden; border-right: none; min-width: 0; }
+main.hide-peek #detail-pane { overflow: hidden; border-left: none; min-width: 0; }
+main.hide-rail .resize-handle[data-panel="rail"] { visibility: hidden; }
+main.hide-peek .resize-handle[data-panel="peek"] { visibility: hidden; }
+
+.resize-handle {
+    cursor: col-resize;
+    background: transparent;
+    transition: background var(--t-fast);
+    z-index: 10;
+}
+
+.resize-handle:hover,
+.resize-handle.active {
+    background: var(--accent);
+    opacity: .4;
+}
 
 /* ── TAG RAIL ───────────────────────────────────────── */
 #tag-rail {