fix(gui): denser Subagents, Usage, and Claude workspaces

[Wibias]

Summary

• Subagents: rail + featured main (models in the rail, less vertical sprawl)
• Usage: section rail (Overview / Models / Providers / Coverage) to cut one-bit scrolling
• Claude Code: section rail; auth dropdown right-aligned; compact effective-auth strip; Fast Mode uses shared Select; sidecar model datalist; fix Background helper [object Object] labels
• Claude Desktop: opaque sticky profile bar; mono model IDs; gap above Search Models

Keeps current fetch/save guards and auth/auto logic from dev.

Test plan

• Subagents workspace rail selects models without a tall stacked list
• Usage sections switch via rail without long single-page scroll for basics
• Claude Code: auth dropdown alignment, Fast Mode select, sidecar autofill, helper labels readable
• Claude Desktop: sticky header opaque; search spacing; mono IDs match Models feel
• Existing GUI tests for subagents/usage/claude still pass

Part of the GUI UI/UX split (UI-2). Companions: #607 (UI-1), #629 (UI-3 Storage).

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