feat(ui): zen mode and panel toggles

- z: toggle zen mode (hide both sidebars)
- [: toggle tag rail
- ]: toggle peek pane
- Panel state persisted in localStorage
- CSS grid transition for smooth collapse

web/style.css

@@ -179,8 +179,16 @@ main {
     display: grid;
     grid-template-columns: 192px 1fr 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.hide-rail #tag-rail { overflow: hidden; border-right: none; }
+main.hide-peek #detail-pane { overflow: hidden; border-left: none; }
+
 /* ── TAG RAIL ───────────────────────────────────────── */
 #tag-rail {
     background: var(--surf);