feat(panels): peek-open side panels with a docked tab

The shared SidePanel now follows the nav rail's model, so Filters, Plex and
Playlists all inherit it: unpinned it sits as a small tab docked flush against
the rail (replacing the full-height vertical strip) and expands into the card on
hover or keyboard focus without pushing content; pinned it stays expanded
in-flow. The collapse toggle became pin/unpin, and panels now default to
unpinned. The tab carries the active-filter count and an accent ring.

The tab stays mounted under the open card on purpose: swapping it out from under
the pointer drops the hover that opened the card, and unmounting it while
focused strands keyboard focus.

Measured: feed content starts at 123px instead of 336px when unpinned.

frontend/src/i18n/locales/en/sidebar.json

@@ -16,8 +16,8 @@
   "hideWidget": "Hide widget",
   "expand": "Expand",
   "collapse": "Collapse",
-  "collapsePanel": "Collapse filters",
-  "expandPanel": "Expand filters",
+  "pin": "Pin panel open",
+  "unpin": "Unpin panel",
   "any": "Any",
   "all": "All",
   "tagModeTooltip": "Match any vs all selected tags",

frontend/src/i18n/locales/hu/sidebar.json

@@ -16,8 +16,8 @@
   "hideWidget": "Modul elrejtése",
   "expand": "Kibontás",
   "collapse": "Összecsukás",
-  "collapsePanel": "Szűrők összecsukása",
-  "expandPanel": "Szűrők kibontása",
+  "pin": "Panel rögzítése nyitva",
+  "unpin": "Panel rögzítésének feloldása",
   "any": "Bármelyik",
   "all": "Összes",
   "tagModeTooltip": "Bármelyik vagy az összes kijelölt címke illeszkedjen",