Frontend: Remove feature flag + simplify shared instances component

[n-lark]

We need to check that the iframe renders correctly in PROD:

✅ If yes - remove feature flag
❌ If no - debug

✅ Confirmed this was working in prod.

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Acceptance criteria

• Confirm the iframe renders in PROD
• Remove posthog feature flag from code EMBEDDED_DASHBOARD_ENABLED
• Retire the dashboardRoleOnly list - Once EMBEDDED_DASHBOARD_ENABLED is removed, the flag-off fallback and its mode in Instances.vue are dead code. Drop them for a single dashboards list.

[n-lark]

Dashboard Cleanup (flag removal + dashboardRoleOnly retirement) — Test Plan

✅ All tests passed!

EMBEDDED_DASHBOARD_ENABLED is gone; the embedded dashboards feature is always on. The TeamInstances dashboard-only mode is retired.

Feature always on (flag removed)

• With the PostHog flag absent/off, team "Dashboards" nav entry still shows
• Application "Dashboards" tab still shows (after Remote Instances)
• Instance "Dashboard" tab shows when the instance has Dashboard 2 installed, hidden when it doesn't
• No console errors referencing isEmbeddedDashboardEnabled / featuresCheck

Dashboard-only user landing

• A dashboard-role-only user visiting /team/:slug lands on the Dashboards list (title "Dashboards"), not Hosted Instances
• Empty state ("There are no dashboards in this team.") shows when they have no dashboards
• Row click opens the immersive viewer; the viewer's drawer lists their dashboards

Hosted Instances regression (Instances.vue)

• Title reads "Hosted Instances"; full helptext (all three paragraphs) renders
• Rows selectable → row click opens the instance
• Search, sort, and pagination work
• Empty state ("Get Started with your First Node-RED Instance") + Create Instance button show when the team has no instances