Add a new full width layout

[cstns]

Description

• Added a new Plain layout that does not include navigation menus.
• Altered the way that the layout is selected on page load, the layout selection is determined based on route 'layout' attribute and in absence will default to the Platform layout.
• Altered the log polling functionality which was based on route paths to again use a meta attribute 'shouldPoll' adding the possibility to fine tune the routes that use it.
• Extracts Alerts logic into a standalone mixin

Related Issue(s)

required in #3656

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

Labels

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

[cstns]

The existing layout functionality should not be impacted as no pages currently use the plain layout. This being said, the setupApp should also be tested.
In order to check the new plain layout, one could add the 'layout: "plain"' meta attribute to a page (one that preferably does not have teleport components) and check that the page loads without any menus. Additionally, alerts and modals should be tested that they work on the new layout.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 48 lines in your changes are missing coverage. Please review.

Project coverage is 42.01%. Comparing base (6b92d2d) to head (3888e69).
Report is 5 commits behind head on main.

Files	Patch %	Lines
frontend/src/mixins/Dialog.js	0.00%	20 Missing ⚠️
frontend/src/mixins/Alerts.js	0.00%	10 Missing ⚠️
frontend/src/App.vue	0.00%	7 Missing ⚠️
frontend/src/layouts/Plain.vue	0.00%	7 Missing ⚠️
...end/src/pages/instance/components/InstanceLogs.vue	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3740      +/-   ##
==========================================
- Coverage   42.04%   42.01%   -0.04%     
==========================================
  Files         631      634       +3     
  Lines       24601    24620      +19     
  Branches     6019     6030      +11     
==========================================
  Hits        10344    10344              
- Misses      14257    14276      +19     

Flag	Coverage Δ
backend	79.14% <ø> (ø)
frontend	1.89% <0.00%> (-0.01%)	⬇️

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[joepavitt]

Approved - just running tests against the new main changes being merged, once they pass, you're good to go