feat: add dark mode support (#962)

[vstudio79]

Summary

Adds proper dark mode / light mode support to app/dashboard and app/portfolio with prefers-color-scheme handling, resolving astera-hq#962.

Changes

Core theming infrastructure

• tailwind.config.ts — Convert all hardcoded brand hex colors to CSS custom properties (var(--gold), var(--card), etc.) so colors automatically adapt to the active theme
• globals.css — Expand CSS variable definitions for both :root (dark) and :root.light (light) including --bg-navy, --amber, --text-primary, --text-secondary. Remove fragile !important light-mode overrides that were used to patch hardcoded brand classes
• ThemeProvider.tsx — Add matchMedia.addEventListener("change") listener so the app follows OS prefers-color-scheme changes when the user has not explicitly toggled the theme

Page-level updates

• app/dashboard/page.tsx — Replace all text-white / hover:text-white with theme-aware text-brand-text / hover:text-brand-text classes
• app/dashboard/credit/page.tsx — Same treatment for inputs, textareas, and hover states
• app/portfolio/page.tsx — Replace text-white with text-brand-text, replace bg-black/20 overlays with bg-brand-navy/40

What this fixes

Before	After
Brand colors hardcoded to dark hex values	Colors resolve via CSS variables, switching per theme
Light mode patched with !important overrides	Clean CSS variable cascade — no overrides needed
No OS preference change listener	Follows prefers-color-scheme in real time
text-white always white regardless of theme	text-brand-text resolves to --text-primary (dark in light mode)

Testing

• Dark mode renders correctly (default)
• Light mode renders correctly via toggle
• OS prefers-color-scheme changes are reflected when no explicit preference is set
• Theme persists across page reloads via localStorage
• ESLint passes (0 errors, pre-existing warnings only)
• TypeScript compiles (pre-existing errors only, none from this PR)