Fix invisible context menus and confirm dialogs (transparent panel background)

[epeicher]

What it does

Restores the opaque dark panel behind <wpd-context-menu>, <wpd-confirm-dialog>, and <wpd-modal>. All three currently render fully transparent — right-clicking a tile in My WordPress shows the menu items as loose white text floating over the window body, and the delete-confirmation prompt is equally unreadable.

Before	After

Rationale

The desktop-themes texture work (#420) added a texture-slot longhand immediately after each surface's background: shorthand:

background: var( --wpd-context-menu-bg, var( --desktop-mode-bg, #1d2327 ) );
background-image: var( --wpd-menu-bg-image, none ); /* texture slot */

--desktop-mode-bg is the wallpaper token and holds a gradient — an <image>, not a <color>. The shorthand therefore routed the entire paint into background-image (leaving background-color: transparent), and the texture slot's none fallback then wiped that image out. With no desktop theme active, nothing painted at all.

#420 converted other surfaces from background: to background-color for exactly this shorthand-resets-the-texture reason; these three were missed because their fallback chain bottoms out in the gradient rather than a color literal.

Implementation

Each surface now splits the paint into its two longhands, so the texture slot owns background-image outright:

background-color: var( --wpd-context-menu-bg, #1d2327 );
background-image: var( --wpd-menu-bg-image, none );

--desktop-mode-bg leaves the chain deliberately: an image-valued token can never feed background-color, and keeping it as the texture slot's fallback would paint the default gradient over any theme-set surface color (the :root gradient is always defined). The literal #1d2327 matches the gradient's base tone, so the default look is unchanged apart from losing an imperceptible tint sweep. A comment at each site documents the constraint.

Testing instructions

1. npm run env:start, then in the desktop shell open My WordPress → Media (any section with tiles works).
2. Right-click a tile — the context menu must render on an opaque dark panel with white items and a red Delete permanently.
3. Pick Delete permanently — the confirmation dialog must render the same opaque panel; Cancel dismisses it.
4. npm run test:js (2353 tests), npm run lint, npm run typecheck — all green.