feat(ui): a visible way into the command palette

Ctrl/Cmd+K and the shortcut sheet had no entry point but a palette row, behind the other
undiscoverable chord. Owner's pick from the S7 mockups (2026-09-19): a key cap at the end of the
header's search field opens the palette, and a page with no search field shows a small "Jump to…"
pill with the same key cap in its place, so the entry is on every page. The cap reads the chord the
palette row binds from the keymap table (Ctrl K, or ⌘ K on a Mac) and stays out of a phone's
header, where there is no keyboard to press it on.

frontend/src/lib/keymap.ts

@@ -443,6 +443,8 @@ export const useNavigablePages = (): readonly Page[] => navigablePages.use();
 const paletteOpen = createStore(false);
 
 export const closePalette = (): void => paletteOpen.set(false);
+/** Open it from a visible control (the header's key cap / launcher), not only from Ctrl/Cmd+K. */
+export const openPalette = (): void => paletteOpen.set(true);
 export const usePaletteOpen = (): boolean => paletteOpen.use();
 
 /** Bind the `global` and `page` scopes, once, for the whole app. It lives here rather than inside