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/ui/Modal.tsx

@@ -32,11 +32,13 @@ export default function Modal({
   onClose: () => void;
   children: ReactNode;
   maxWidth?: string;
-  /** `drawer` slides in from the right edge and runs full height; everything else about the shell —
-   *  the layer stack, ESC, the focus trap, Back-to-close, the scroll lock — is identical. A drawer
-   *  is a placement, not a second dialog implementation: duplicating that behaviour is exactly how
-   *  two shells drift apart. */
-  placement?: "center" | "drawer";
+  /** `drawer` slides in from the right edge and runs full height; `top` pins the panel near the top
+   *  of the viewport (where a search-first dialog like the command palette belongs, so the list grows
+   *  downwards instead of the whole thing jumping as results change). Everything else about the shell
+   *  — the layer stack, ESC, the focus trap, Back-to-close, the scroll lock — is identical. A
+   *  placement is not a second dialog implementation: duplicating that behaviour is exactly how two
+   *  shells drift apart. */
+  placement?: "center" | "drawer" | "top";
 }) {
   const { t } = useTranslation();
   // Browser/mouse Back closes the topmost modal instead of navigating away.
@@ -114,10 +116,15 @@ export default function Modal({
   }, [restoreTo]);
 
   const drawer = placement === "drawer";
+  const top = placement === "top";
   return createPortal(
     <div
       className={`fixed inset-0 z-overlay flex bg-black/70 overlay-backdrop ${
-        drawer ? "justify-end" : "items-center justify-center p-4 sm:p-6"
+        drawer
+          ? "justify-end"
+          : top
+            ? "items-start justify-center p-4 pt-[8vh] sm:p-6 sm:pt-[10vh]"
+            : "items-center justify-center p-4 sm:p-6"
       }`}
       onMouseDown={(e) => {
         pressedOnBackdrop.current = e.target === e.currentTarget;