Use neutral workspace chrome in Theme Preview

[brsbl]

What changed

• Replace the fixed account label with Workspace.
• Add the marketplace overview for palette comparison and theme iteration.

Verification

• Remote CI passed all 40 checks, including the tracked preview bundle.
• Isolated upstream bb refuses the local ref because theme-preview is a reserved bundled plugin ID.
• Standalone UI smoke verification is blocked; the live marketplace reserved-ID list also prevents this new listing.

Screenshots needed

• Preview chrome: open Theme Preview in a fresh isolated bb with a synthetic project named Sample project; show the Thread view with Workspace in the preview sidebar.
• Theme comparison: select bundled light and dark themes; capture the theme picker and token/contrast specimens. Use no personal threads or custom theme names.

Capture is assigned to the separate screenshot worker. Use real plugin surfaces, synthetic data, at least 1200 px width, and files no larger than 2 MiB.

BB-Thread-ID: thr_wfy26bwmi8

AGENT GENERATED

[brsbl]

Theme Preview ships bundled with bb, so it won't be listed in the marketplace. This fix moved to the bundled copy in get-bb/bb#5728.