Fix illegible text fields inside wpd-modal dialogs

[epeicher]

What it does

Makes text fields inside <wpd-modal> dialogs legible again. The "Upload to Media Library" drag-and-drop dialog rendered its pre-filled Title / Filename / Alt text values as near-white text on white inputs.

Before	After

Rationale

<wpd-modal>'s dialog surface is dark and re-points the shared kit tokens (--desktop-mode-text, --desktop-mode-border, the muted colors) so slotted wpd-* controls resolve dark-surface colors, but it never re-pointed --desktop-mode-window-bg. That token is the input background for <wpd-text-field> / <wpd-textarea>, so it kept its :root value of #fff (windows are light) while the text inside resolved to the modal's light #f0f0f1 — invisible values.

Implementation

Added the missing token to the existing dark-token block on the modal :host:

--desktop-mode-window-bg: #2c3338;

Solid rather than translucent on purpose: wpd-menu and wpd-multiselect popovers read the same token and must stay opaque over slotted content. Because the fix is at the modal level, it also repairs every other modal hosting form fields (folder share settings, wallpaper renderConfig dialogs, the games challenge dialog). The share-settings capability control keeps its own instance-level inline overrides, which still win.

The static help summary on WpdModal now lists -window-bg among the re-pointed tokens.

Testing instructions

1. Drag a file from your OS onto the desktop shell.
2. In the "Upload to Media Library" dialog, the Title / Filename / Alt text / Caption / Description fields should render dark inputs with legible pre-filled text.
3. npm run test:js stays green (2051 tests).