fix(theme): restore PrimeNG v21 Aura dark theme — fixes broken UI

[wcypierre]

Problem

The Angular 13→21 + PrimeNG 13→21 migration removed `primeng/resources/themes/bootstrap4-dark-blue/theme.css` from `angular.json` (correctly — the path no longer exists in PrimeNG v17+), but never configured a replacement theme. This broke the UI in two ways:

1. CSS variables undefined — `styles.scss` and `app.component.scss` use `--surface-b`, `--text-color`, `--font-family` which were defined by the old theme. Without them, the browser falls back to white background, black text, system font.
2. PrimeNG v21 component tokens undefined — PrimeNG v21 components use `dt()` design tokens that resolve to `--p-*` CSS variables. These are only injected when `providePrimeNG({ theme: ... })` is called. Without it, all buttons, dropdowns, dialogs, and progress bars render as invisible/unstyled elements.

Fix

• Install `@primeuix/themes` (the PrimeNG v21 theme preset package)
• Configure `providePrimeNG` with the Aura dark preset in `AppModule`
• Add `class="p-dark"` to `` to permanently activate dark mode
• Add `:root` CSS variables restoring the original bootstrap4-dark-blue values (`--surface-b`, `--text-color`, `--font-family`)