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/history.dom.test.ts

@@ -11,7 +11,7 @@
 // no reconcile-driven history.back() runs and the shared module state stays clean between tests.
 import { describe, it, expect, vi, afterEach } from "vitest";
 import { renderHook, act } from "@testing-library/react";
-import { useBackToClose, useBackToExit } from "./history";
+import { closeThenNavigate, useBackToClose, useBackToExit } from "./history";
 
 // Let the mount effect's coalesced reconcile microtask settle.
 const settle = () =>
@@ -87,3 +87,28 @@ describe("useBackToExit back-ladder", () => {
     rg.unmount();
   });
 });
+
+describe("closeThenNavigate", () => {
+  // The bug it exists for: an overlay's unmount reclaims its Back-to-close entry with a programmatic
+  // history.back(). A pushState made before that lands is undone by it, so a dialog command that
+  // navigates does nothing but close. PlayerModal hit it once; the command palette hit it again.
+  it("closes first and navigates only after the reclaim lands", () => {
+    const order: string[] = [];
+    closeThenNavigate(
+      () => order.push("close"),
+      () => order.push("navigate"),
+    );
+    expect(order).toEqual(["close"]);
+
+    window.dispatchEvent(new PopStateEvent("popstate"));
+    expect(order).toEqual(["close", "navigate"]);
+  });
+
+  it("navigates exactly once, however many pops follow", () => {
+    const navigate = vi.fn();
+    closeThenNavigate(() => {}, navigate);
+    window.dispatchEvent(new PopStateEvent("popstate"));
+    window.dispatchEvent(new PopStateEvent("popstate"));
+    expect(navigate).toHaveBeenCalledTimes(1);
+  });
+});