feat: [1679] Add multiple groups per tab

[n-lark]

Description

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Pages using the Tabs layout can show multiple groups in one tab.

• Pages get a Tabs list and groups get a Tab dropdown.
• Groups sharing a tab render side-by-side like the Grid layout. Existing flows are unchanged.
• The selected tab survives redeploys and falls back to the first tab if hidden.
• [1679] Add multiple groups per tab to the page layout  #2260 is the work to add the tabs to the layout config panel, 2nd layer of stack.

Test plan

✅ All tests passed

• Existing Tabs page with no page tabs renders unchanged (one group per tab, no titles, order 0 first)
• Groups sharing a tab render side-by-side in group order; single-group tabs look as before
• Tabs list shows only for the Tabs layout, with a hint while empty
• Renaming a tab keeps its id; new tabs get unique ids
• Group dialog lists the page's tabs; moving to a Grid page hides and clears the field
• Hiding every group in the active tab via ui-control selects another tab
• Redeploying keeps the selected tab
• Multi-group tab matches Grid layout at desktop, tablet and mobile widths

Related Issue(s)

Resolves #1679

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

[cstns]

Took this for a spin locally (dashboard + editor dialogs + the new Cypress specs), all working as described. Left a few inline thoughts, none of them blockers.