themes get their own control, and the vimazing keys

Fourteen themes was too many to keep under an avatar next to "Sign out" — that is where
a preference goes when nobody has decided it matters. It now has its own button in the
header, between the controls and the avatar, grouped by family.

── The keys, ported from the owner's vimazing ──

  ctrl+shift+l / ctrl+shift+h   cycle forward / back, from anywhere, no UI
  ctrl+alt+space                open the list
  j / k                         move the focus
  l or space                    apply the focused theme to the app BEHIND the list
  h                             back to where you started
  enter                         keep what you are looking at
  escape                        restore and close

The browse is the point, and it is why the overlay is a portal over a 55% backdrop
rather than a modal: you are not reading names, you are watching the application change.
The theme you started on is captured when the list opens, so `h` and `escape` undo a
whole browse however many previews happened inside it.

Typing is still typing: the list ignores j/k/l/h while focus is in an input, textarea,
select or anything contenteditable.

── Where it lives ──

The key handling is in the theme package — it is behaviour that belongs to themes, and
it needs no UI. The list and the button are Officer's, because a dialog would drag a
component library into a package that currently has none.

Verified by driving the real keyboard in a browser:

  start duck-dark · ctrl+shift+l -> plain · ctrl+alt+space -> overlay
  j, l -> tokyo-night (previewed) · escape -> plain, closed

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/workspaces/components/ui/ThemeProvider.tsx

@@ -12,5 +12,5 @@
  *
  * New code should import from `theme` directly. This file exists to be deleted once nothing points here.
  */
-export { ThemeProvider as ColorModeProvider, ThemeProvider, useTheme, useColorMode, useThemeAsset } from 'theme';
+export { ThemeProvider as ColorModeProvider, ThemeProvider, useTheme, useColorMode, useThemeAsset, useThemeKeys } from 'theme';
 export type { Theme, ThemeId } from 'theme';