fix: disable non-current projects in switcher when NAO_DEFAULT_PROJECT_PATH is set

[Nithishvb]

Closes: #1172

When self-hosting with NAO_DEFAULT_PROJECT_PATH set, the project switcher in settings silently ignored user selections — clicking a different project did nothing with no feedback.

## Changes

• Backend (system.routes.ts): Exposed hasDefaultProjectPath in the getPublicConfig endpoint so the frontend knows when the env var is set.
• Frontend (project-selector.tsx): Added optional disabledProjectIds prop to ProjectSelector. When provided, matching items are rendered with Radix's native disabled attribute (greyed out, not clickable).
• Frontend (sidebar-settings-nav.tsx): Computes disabledProjectIds from the isProjectPinned flag and passes it to ProjectSelector.
• Frontend (sidebar.tsx): Derives isProjectPinned from config (!isCloud && hasDefaultProjectPath) and threads it to the settings nav.

Now the dropdown remains openable so users can see all projects, but non-current entries are visibly disabled with no interaction possible — making the constraint obvious instead of silently doing nothing

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