feat(e10): a jump-to palette on Ctrl/Cmd+K

Reaching a channel three modules away cost a rail, a page and a scroll.
Now it costs a keystroke and half a word: Mod+K searches modules,
channels, playlists, the saved views applicable where you stand, and the
app-wide actions.

Scope agreed with the product owner: navigation + actions, but only the
reversible local ones (the shortcuts sheet, the theme). "Sync from
YouTube" is deliberately absent — it is a long-running job against a
quota, it wants the progress and error surface the Channels page already
gives it, and one mistyped Enter is a worse failure than the walk it
would have saved.

Matching is word-wise and accent-folded, because this is a bilingual app
and nobody reaches for the accent keys in a hurry: "pod thevr" finds
TheVR Podcast and "penzugy" finds pénzügy. The arrows walk one flat
result list so crossing a group heading is invisible, and the field
carries the combobox/listbox aria tree that virtual selection needs.

ui/CommandPalette is domain-free (items are a prop, per the ui/ import
rule); AppCommandPalette supplies them. The shell is Modal again, with a
new `top` placement so a search-first dialog grows downwards instead of
jumping as results change.

Also fixes a bug the palette hit on its first run, and that PlayerModal
had already hit once: closing a dialog reclaims its Back-to-close
history entry with a programmatic history.back(), which undoes any
pushState made before it lands — so a command that navigates only closed
the palette. The rule is now closeThenNavigate() in history.ts with a
test, and PlayerModal's inline copy is gone.

The global `h` moves out of KeyboardHelp into useGlobalKeymap beside
Mod+K, so one listener owns the whole `global` scope and dispatch's
exhaustiveness keeps meaning what it says.

frontend/src/lib/keymap.ts

@@ -51,6 +51,7 @@ export const SHORTCUTS = [
   // --- global ---------------------------------------------------------------------------------
   // `h`, not `?`: on a Hungarian layout `?` is a Shift chord, and the one help key people already
   // have in their fingers here is the Plex player's `h`. Making it app-wide costs no relearning.
+  { id: "palette", scope: "global", keys: ["Mod+k"], labelKey: "keys.palette" },
   { id: "help", scope: "global", keys: ["h"], labelKey: "keys.help" },
   // Matched by `useContextMenu` on the focused row, not by a window-level dispatch: which row the
   // menu belongs to is a property of focus, not of the app.
@@ -242,6 +243,36 @@ export const toggleKeyboardHelp = (): void => helpOpen.set((v) => !v);
 /** Subscribe to whether the sheet is showing (the Plex player suppresses wheel-volume under it). */
 export const useKeyboardHelpOpen = (): boolean => helpOpen.use();
 
+// ── The command palette ───────────────────────────────────────────────────────────────────────
+const paletteOpen = createStore(false);
+
+export const closePalette = (): void => paletteOpen.set(false);
+export const usePaletteOpen = (): boolean => paletteOpen.use();
+
+/** Bind the `global` scope, once, for the whole app. It lives here rather than inside either
+ *  overlay because both are opened by it: a listener that sat in one of them would have to route a
+ *  key belonging to the other, and `dispatch`'s exhaustiveness would stop meaning anything. App
+ *  calls this; the overlays are pure renderers of their store. */
+export function useGlobalKeymap(): void {
+  useEffect(() => {
+    const onKey = (e: KeyboardEvent) => {
+      dispatch(e, "global", {
+        help: (ev) => {
+          ev.preventDefault();
+          helpOpen.set((v) => !v);
+        },
+        palette: (ev) => {
+          // Mod+K is the browser's "focus the search bar" in a few builds; we want it.
+          ev.preventDefault();
+          paletteOpen.set((v) => !v);
+        },
+      });
+    };
+    window.addEventListener("keydown", onKey);
+    return () => window.removeEventListener("keydown", onKey);
+  }, []);
+}
+
 /** Answers for the `when` conditions, published by the surface that knows them. */
 export type HelpConditions = { readonly [K in ShortcutWhen]?: boolean };
 const helpConditions = createStore<HelpConditions>({});