Feat/implement dark mode system for merchant dashbaord

[Ekene001]

closes #88

Dark Mode System for Merchant Dashboard

Implements a full production-ready dark mode system for the merchant dashboard with system preference detection, manual toggle, localStorage persistence, and CSS variable-based theming across all components.

Changes

Theme Infrastructure

• Extended ThemeProvider to support three modes: light, dark, and system (auto-detects OS preference via matchMedia)
• Theme persisted to localStorage under useroutr-theme key
• Added toggleTheme() cycling light → dark → system, plus setPreference() for direct selection
• Added inline flash-prevention <script> in root layout.tsx to apply theme class before first paint

CSS Token System (globals.css)

• Added dark mode semantic tokens: --bg-page, --bg-card, --bg-sidebar, --text-primary, --text-secondary, --border-color, --input-bg, --input-border, --code-bg
• Added chart tokens: --chart-tick, --chart-grid, --chart-tooltip-bg, --chart-tooltip-text, --chart-tooltip-border
• Added status tokens: --status-success-bg/text, --status-warning-bg/text, --status-error-bg/text, --status-info-bg/text, --status-purple-bg/text, --status-amber-bg/text
• Registered all tokens in @theme inline block for Tailwind v4 utility class support
• Defined :root (dark-first) and .light overrides

Theme Toggle UI

• Updated site-header.tsx and Topbar.tsx with 3-way toggle button (Sun / Moon / Monitor icons from lucide-react)
• Each mode shows tooltip label and appropriate aria-label

Component Fixes (removed all hardcoded colors)

• [AnalyticsDashboard.tsx] — chart grid, axis, tooltip, bar fills, heatmap all use CSS vars
• InvoiceDetailSheet.tsx — status badges use bg-status-* / text-status-* tokens
• InvoicesTable.tsx — same status badge token migration
• InvoiceDrawer.tsx — discount color uses status token
• MetricCard.tsx — delta badges and sparkline strokes use status tokens
• GreetingHeader.tsx — live indicator uses text-status-success-text
• (auth)/layout.tsx — gradient background uses CSS vars instead of rgba()
• QRCodeModal.tsx / LinkCreatedModal.tsx — bg-white → bg-card