[7165 + 7164 + 7163 + 7162] Wire device-updated / assign-device events into the immersive remote-instance editor

[n-lark]

Description

This wraps the following changes into one PR:

1. Follow Up: Remote Instance group assignment UI does not refresh in immersive mode #7163
2. Follow Up: Remote Instance application assignment broken in immersive mode #7162
3. Follow Up: Remote Instance save settings button stays enabled after saving environment variable in immersive mode #7164
4. Follow Up: Remote Instances Save Settings button stays enabled after saving max HTTP payload size in immersive mode #7165

✅ All tests passed!

• 7162 immersive Assign (unassigned device): unassigned remote instance → immersive editor → Settings → click Assign. Dialog opens; pick Instance or Application and confirm. Device updates without manual reload; Assign UI flips to assigned state.
• 7162 Unassign → Assign cycle: assigned remote instance → immersive Settings → Unassign. Assign button re-enables and is responsive immediately (no stale state, no manual reload).
• 7163 group Remove + Reassign: application-owned remote instance currently in a device group → immersive Settings → General → in the Group section, click Remove from the group. Section flips to "no group" state immediately. Then click Reassign (or Assign to a group) and pick a group — section updates without manual reload.
  • Known limitation: group removal clears the active snapshot → editor.connected briefly false → pre-existing watcher in Editor/index.vue:213-227 bounces to device-overview. Out of scope for this bug fix.
• 7164 env-var save: immersive Settings → Environment → add or modify a var → Save Settings. Button disables after success (no longer stays "dirty").
• 7165 max-payload save: immersive Settings → Editor → change max HTTP payload size → Save Settings. Button disables after success.
• Non-immersive (no-regression): repeat Follow Up: Remote Instance application assignment broken in immersive mode #7162-Follow Up: Remote Instances Save Settings button stays enabled after saving max HTTP payload size in immersive mode #7165 flows from non-immersive Settings — behavior unchanged from main.

Related Issue(s)

Resolves #7163
Resolves #7162
Resolves #7164
Resolves #7165

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 76.61%. Comparing base (424fbb5) to head (418ee32).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

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  Coverage   76.61%   76.61%           
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  Files         405      405           
  Lines       20427    20427           
  Branches     4925     4925           
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  Hits        15650    15650           
  Misses       4777     4777           

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