Add light/dark mode switcher to all web projects

[thusser]

Summary

All pyobs web projects (pyobs-web-client, pyobs-web-admin, pyobs-robotic-backend, pyobs-archive, pyobs-pipeline, and any other browser-based UI) need a light/dark mode switcher.

Notes

• Should apply consistently across all web projects, not just one.
• Needs a decision on shared implementation (e.g. common CSS variables/theme approach) vs. per-project implementation.

[thusser]

Light/dark mode switcher implemented in pyobs-archive, pyobs-web-admin, pyobs-robotic-backend, and pyobs-pipeline (theme-aware CSS custom properties, toggle persisted to localStorage, falls back to OS preference).

pyobs-web-client intentionally deferred: its stack is different (Vue/Canvas), and several chart components (TimeSeriesChart, DistanceChart, FocusCurveChart, OffsetScatterChart, OffsetMagnitudeChart) draw colors directly via Canvas 2D (ctx.fillStyle/strokeStyle), which CSS variables can't reach — would need each chart to redraw on theme change. Not done here; tracking as follow-up work whenever picked back up.