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/i18n/locales/en/keys.json

@@ -19,5 +19,6 @@
   "stats": "Stats for nerds",
   "back": "Stop & back to feed",
   "browseBack": "Back one level",
-  "closePlayer": "Close the player"
+  "closePlayer": "Close the player",
+  "palette": "Open the jump-to palette"
 }

frontend/src/i18n/locales/en/palette.json

@@ -0,0 +1,15 @@
+{
+  "title": "Jump to…",
+  "placeholder": "Search modules, channels, playlists…",
+  "empty": "Nothing matches.",
+  "theme": "Theme: {{name}}",
+  "videoCount": "{{count}} video",
+  "videoCount_other": "{{count}} videos",
+  "groups": {
+    "modules": "Modules",
+    "channels": "Channels",
+    "playlists": "Playlists",
+    "views": "Saved views",
+    "actions": "Actions"
+  }
+}

frontend/src/i18n/locales/hu/keys.json

@@ -19,5 +19,6 @@
   "stats": "Statisztika (nerdöknek)",
   "back": "Leállítás és vissza a feedre",
   "browseBack": "Vissza egy szintet",
-  "closePlayer": "Lejátszó bezárása"
+  "closePlayer": "Lejátszó bezárása",
+  "palette": "Ugrás-paletta megnyitása"
 }

frontend/src/i18n/locales/hu/palette.json

@@ -0,0 +1,15 @@
+{
+  "title": "Ugrás…",
+  "placeholder": "Modul, csatorna, lista keresése…",
+  "empty": "Nincs találat.",
+  "theme": "Téma: {{name}}",
+  "videoCount": "{{count}} videó",
+  "videoCount_other": "{{count}} videó",
+  "groups": {
+    "modules": "Modulok",
+    "channels": "Csatornák",
+    "playlists": "Listák",
+    "views": "Mentett nézetek",
+    "actions": "Műveletek"
+  }
+}