Add dark mode and fix the theme built from ThemeData.fallback

Problem

Raised in the placement meeting. There is no dark mode, and the theming is not in a state where adding one is a small change.

The theme is built with Theme.of(context).copyWith(...) where the context sits above MaterialApp, so the base is ThemeData.fallback() rather than a real theme. There is no darkTheme and no themeMode. Separately, 44 hardcoded colour literals in lib/ would not follow a theme even if one existed.

Where

• lib/main.dart:20-44 (theme built from a context above MaterialApp)
• Examples of hardcoded colours: lib/views/CandidateDetailsView.dart:211, lib/views/CompanyDetailView.dart:216

Expected

Light and dark both work and follow the system setting.

Fix sketch

• Replace the theme with ThemeData(colorScheme: ColorScheme.fromSeed(seed: R.primaryCol)), and add darkTheme plus themeMode: ThemeMode.system.
• Replace the colour literals with colorScheme and textTheme lookups.
• Check lib/themes/theme.dart on the unmerged origin/beta branch before rewriting this from scratch; it may already be most of the work.

Start with lib/main.dart:20-44 to inspect how the theme is built, then check lib/themes/theme.dart on the unmerged origin/beta branch before changing it. Search lib/ for the 44 hardcoded colour literals, including lib/views/CandidateDetailsView.dart:211 and lib/views/CompanyDetailView.dart:216. Done means light and dark themes follow the system setting and colours use theme lookups.