Adds a full dark/light theme system across all frontend views and components.
Closes#340
- Add `theme.css` with CSS custom properties for light/dark color tokens
- Add `ThemeToggle.vue` component (sun/moon button) with localStorage persistence
and prefers-color-scheme fallback
- Update `LanguageSwitcher.vue` to use `color: inherit` so it adapts to
both dark navbars (Home, Process) and light headers (workflow views)
- Apply ThemeToggle and CSS variable theming to all views:
Home, Process, MainView, InteractionView, SimulationView,
SimulationRunView, ReportView
- Update all workbench components to replace hardcoded colors with
CSS variables: GraphPanel, Step1GraphBuild, Step2EnvSetup,
Step3Simulation, Step4Report, Step5Interaction
- Pin Python version to 3.12 via `.python-version` for uv compatibility
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Gloris Denisse