Add feature flag and scaffold for Projects summary widget

What does this MR do and why?

Adds the scaffolding for a new Projects summary widget on the group Security Inventory page, to the left of the existing Tool coverage widget, behind a new feature flag that defaults to disabled. No behavior change is visible yet; this is the first of a stack of small MRs that build the widget incrementally.

A new feature flag, security_inventory_projects_widget, gates a new ProjectsCard component. The component currently renders only a title and a GlSkeletonLoader placeholder, styled as a bordered panel (gl-border gl-rounded-base gl-p-5, matching the design) rather than a GlCard. inventory_dashboard.vue now wraps ProjectsCard and ToolCoverageCard in a flex row, with ProjectsCard rendered only when glFeatures.securityInventoryProjectsWidget is on.

References

• Issue: Add feature flag and scaffold for Projects summ... (#632828)
• Design: Security coverage prototype
• Design issue: #527861
• Rollout issue: #632851

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the feature flag in a Rails console: Feature.enable(:security_inventory_projects_widget).
2. Visit a group's Secure > Security inventory page.
3. Confirm a bordered Projects panel with a skeleton loader appears to the left of the Tool coverage widget.
4. Disable the flag again (Feature.disable(:security_inventory_projects_widget)), reload, and confirm the page looks exactly as it did before this MR (no Projects panel).

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