Add dark/light mode theme toggle that actually works

[Wilfred007]

What

ThemeToggle.tsx component exists but is not wired to a persistent theme state. CSS variables for dark/light themes are incomplete. Wire the toggle to zustand store, persist preference in localStorage, and complete CSS variable sets.

Why

Users expect theme preference to persist. The toggle component exists but doesn't function — it's dead code.

Scope

In scope: Wire ThemeToggle to zustand store, persist in localStorage, complete CSS variable sets for both themes
Out of scope: System theme detection, per-component theme overrides

Acceptance Criteria

• Toggle switches between dark and light mode
• Preference persists across page reloads (localStorage)
• All components render correctly in both themes
• No hard-coded colors that ignore theme variables

Technical Context

• frontend/src/components/ThemeToggle.tsx — existing toggle
• frontend/src/hooks/useTheme.ts — theme hook
• frontend/src/index.css — CSS variables

[waterWang]

I will work on this issue. Claiming it now.

[waterWang]

PR submitted: #579 — feat: wire theme toggle to zustand store with persisted preference