use PatternFly light, dark, and system themes

[jkandasa]

Summary

Replace data-repository custom GUI themes (labels.gui_theme) with PatternFly light, dark, and system.

• Theme is stored in the browser as localStorage key mc-theme and applied before React paints.
• Header (left to right): compact joined theme group (desktop / sun / moon) → locale (🇬🇧 EN-UK) → notifications → help → user.
• Locale menu: flag + name, with EN-UK / TA-IN as secondary text. Light-theme list uses explicit dark text so names are visible.
• Labels, forms, topology, and widgets follow PatternFly colour tokens in dark mode.
• Dashboard widget tiles no longer have a grey fill.

Test plan

• Header theme group: System / Light / Dark. Selected button is solid blue on the dark bar.
• Refresh after choosing Dark; page stays dark (localStorage.mc-theme === "dark").
• System follows OS colour scheme and updates if the OS changes.
• Locale dropdown shows 🇬🇧 EN-UK; list shows names in light theme (not white-on-white).
• Details page labels: pale-blue key, solid-blue value with white text (light); readable chips in dark.
• Dashboard has no grey canvas or grey widget fill.
• Login page follows the same theme.