Use neutral workspace labels in Theme Preview

[brsbl]

Human comments

What was wrong

Theme Preview's desktop sidebar and mobile navigation showed a fixed repository name in their sample workspace chrome.

What changed

• Both preview headers now show the neutral label “Workspace”.

How you verified

• Remote CI passed on 691a4a28, including Theme Preview tests on Linux, Windows, and the exported plugin.
• Before/after captured in a fresh isolated bb (bundled Theme Preview, synthetic preview content).

Screenshots needed

• Desktop: open bundled Theme Preview in a fresh isolated bb; show the Thread preview's sidebar header before and after.
• Mobile: select the narrow preview and open its navigation; show the workspace header before and after.
• Use only the plugin's synthetic preview content and a project named Sample project; no personal accounts or content.

Screenshots

Screen / state	Viewport	Before	After
Thread preview, sidebar header	Desktop · 1440×900 @2×
Narrow preview, navigation open	Mobile @2×

BB-Thread-ID: thr_wfy26bwmi8

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