feat: extract shared GUI switch module, add prefetch and view transitions}
- Add src/core/gui-switch.js shared module for framework switching logic - Replace duplicated switchGui implementations in Angular, React, and Vue login and settings pages with the shared module - Prefetch other GUI framework assets (HTML, scripts, styles) on layout mount in all three frontends with a 3-second delay - Add CSS @view-transition for smooth crossfade animation (400ms) when switching between Angular, React, and Vue - Add inline color-scheme meta script to all three index.html files to prevent flash of unstyled content on initial load - Restyle GUI toggle buttons: use primary theme color for active state, add text-shadow on active framework icon, reduce horizontal padding - Simplify Vue KeyTimeseries aggregation select using computed items array instead of custom slot templates - Fix Vue SettingsPage donate button to use helper instead of direct window.open call - Update description from "dual frontend" to "triple frontend" across package.json and README - Bump version to v2026.4.436