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.ts

@@ -187,6 +187,27 @@ function ensureOverlayListener(): void {
   });
 }
 
+/** Close an overlay that holds a `useBackToClose` entry and THEN navigate — in that order, waiting
+ *  for the browser to confirm it.
+ *
+ *  The trap: closing unmounts the overlay, whose cleanup reconciles the history by issuing a
+ *  programmatic `history.back()`. A `pushState` made before that lands (i.e. anything `setPage` /
+ *  `openChannel` does synchronously) is simply undone by it, so the navigation silently does
+ *  nothing. PlayerModal hit this first — clicking a video's channel name only closed the player —
+ *  and the command palette hit it again the day it was written, which is why the fix lives here
+ *  instead of being re-derived at each call site.
+ *
+ *  Only call this from INSIDE an overlay that uses `useBackToClose`: the popstate we wait for is
+ *  that overlay's own entry being reclaimed, and without one it never arrives. */
+export function closeThenNavigate(close: () => void, navigate: () => void): void {
+  const once = () => {
+    window.removeEventListener("popstate", once);
+    navigate();
+  };
+  window.addEventListener("popstate", once);
+  close();
+}
+
 /** While the calling component is mounted, Back (or the browser/mouse back button) closes it via
  * `onClose` instead of navigating away. Mount the component only while the overlay is open. */
 export function useBackToClose(onClose: () => void): void {