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.test.ts

@@ -143,11 +143,15 @@ describe("dispatch", () => {
     const calls: string[] = [];
     // The handler map is the global scope's routable ids only — `contextMenu` is absent from it,
     // which is the compile-time half of the same claim; tsc rejects passing one.
-    const handlers = { help: () => calls.push("help") };
+    const handlers = {
+      help: () => void calls.push("help"),
+      palette: () => void calls.push("palette"),
+    };
     expect(dispatch(press({ key: "F10", shiftKey: true }), "global", handlers)).toBe(false);
     expect(dispatch(press({ key: "ContextMenu" }), "global", handlers)).toBe(false);
     expect(dispatch(press({ key: "h" }), "global", handlers)).toBe(true);
-    expect(calls).toEqual(["help"]);
+    expect(dispatch(press({ key: "k", ctrlKey: true }), "global", handlers)).toBe(true);
+    expect(calls).toEqual(["help", "palette"]);
   });
 
   it("ignores keys belonging to another scope", () => {
@@ -203,7 +207,7 @@ describe("helpLines", () => {
   it("orders scopes as asked, not as tabled", () => {
     const first = (scopes: Parameters<typeof helpLines>[0]) => helpLines(scopes)[0]?.labelKey;
     expect(first(["plexBrowse", "global"])).toBe("keys.browseBack");
-    expect(first(["global", "plexBrowse"])).toBe("keys.help");
+    expect(first(["global", "plexBrowse"])).toBe("keys.palette");
   });
 });