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/components/PlayerModal.tsx

@@ -45,7 +45,7 @@ import {
 import { renderDescription } from "../lib/descriptionLinks";
 import { fullscreenElement } from "../lib/fullscreen";
 import { dispatch, isActivationTarget, useKeyScope } from "../lib/keymap";
-import { useBackToClose } from "../lib/history";
+import { closeThenNavigate, useBackToClose } from "../lib/history";
 import {
   useNativeControlsYield,
   useYtBigView,
@@ -756,16 +756,11 @@ export default function PlayerModal({
                     onClick={() => {
                       const cid = active.channel_id;
                       const cname = active.channel_title ?? undefined;
-                      // Closing the player pops its own history entry (useBackToClose → history.back()).
-                      // Open the channel only AFTER that pop's popstate — pushing the channel entry
-                      // before it would let the back remove it again (the bug: clicking the name only
-                      // closed the player). One-shot listener fires after the popstate handler.
-                      const open = () => {
-                        window.removeEventListener("popstate", open);
-                        openChannel(cid, cname);
-                      };
-                      window.addEventListener("popstate", open);
-                      onClose();
+                      // Close first, open the channel only once this player's own Back-to-close
+                      // entry has been reclaimed — otherwise that reclaim undoes the push and
+                      // clicking the name merely closes the player. The rule lives in history.ts
+                      // now; the command palette needs exactly the same one.
+                      closeThenNavigate(onClose, () => openChannel(cid, cname));
                     }}
                     className="font-medium hover:text-accent truncate text-left"
                   >