[7891] Frontend: Remove feature flag + simplify shared instances component

[n-lark]

Description

This removes the EMBEDDED_DASHBOARD_ENABLED feature flag and cleans up the dashboardRoleOnly list for Instances.vue.

See test plan here: #7891 (comment)

Related Issue(s)

Resolves #7891

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.72%. Comparing base (eb9911b) to head (5b97185).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7909   +/-   ##
=======================================
  Coverage   75.72%   75.72%           
=======================================
  Files         434      434           
  Lines       23198    23198           
  Branches     6161     6161           
=======================================
  Hits        17567    17567           
  Misses       5631     5631           

Flag	Coverage Δ
backend	75.72% <ø> (ø)

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