[454 + 1538] Default theme presets + fixes for dark themes

[n-lark]

Description

• Five built-in themes — Light, Dark, Dracula, Nord, Sepia — each a complete, accessible (WCAG AA) colour scheme. New dashboards start on Light; pick any from a dropdown, and editing a colour branches off into Custom.
• Dark themes that are usable. Before, a dark palette left light-styled inputs, black-on-black native date pickers, and washed-out charts, gauges and markdown. Now everything follows the theme — legible end to end, on any dark theme you build, not just the presets.

Test plan

• A new dashboard defaults to the Light preset.
• The sidebar dropdown lists all five and creates the chosen one; the ui-theme editor switches a theme and drops to Custom on any colour edit.
• Dark showcase pages: text, charts, gauges, markdown, and the native date picker are all legible.
• npm test passes (all five meet WCAG AA; defaults equal the light preset).

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Related Issue(s)

Resolves #454
Resolves #1538
Resolves https://github.com/FlowFuse/engineering/issues/244

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