feat: scope project view to dashboards while View As is active

[royendo]

While impersonating a user via "View As", scope the project view to the dashboards experience only. Other project tabs (Home, AI, Reports, Alerts, Status, Settings) are hidden, and direct URL access to those routes redirects to /-/dashboards.

• ProjectTabs.svelte collapses to a single Dashboards tab when viewAsUserStore is set.
• The project layout adds a redirect effect that bounces non-dashboard project pages to /-/dashboards while View As is active.
• Per-resource access policies on dashboards are already enforced via the impersonated user's runtime JWT — no change needed there.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

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Developed in collaboration with Claude Code