fix : add dark-mode theming and remove emojis from OnboardingModal

[tmdeveloper007]

Summary of What Has Been Done

Replaced all hardcoded dark-mode hex colors in OnboardingModal with Tailwind dark mode variants. Replaced all emoji characters with Lucide React icon components. Added comprehensive ARIA accessibility attributes.

Changes Made

• Replaced hardcoded bg-[#141f2b] with bg-white dark:bg-slate-900
• Replaced hardcoded text-[#e2eaf2] with text-slate-900 dark:text-slate-100
• Replaced hardcoded text-[#7a9bb5] with text-slate-500 dark:text-slate-400
• Replaced hardcoded text-[#c5d8e8] with text-slate-800 dark:text-slate-200
• Replaced hardcoded text-[#5a7a92] with text-slate-400 dark:text-slate-500
• Replaced hardcoded border-[#1e3a4a] with border-slate-200 dark:border-slate-700
• Replaced hardcoded bg-[#0f1923] with bg-slate-50 dark:bg-slate-800
• Replaced hardcoded bg-[#1e2e3d] with bg-slate-100 dark:bg-slate-800
• Replaced hardcoded border-[#1e2e3d] with border-slate-200 dark:border-slate-700
• Replaced hardcoded border-[#1e3a4a] with border-slate-200 dark:border-slate-700
• Added aria-label="Skip onboarding tour" to skip button
• Added aria-label="Go to previous step" to back button
• Added aria-label="Go to next step" to next button
• Added aria-label="Start using DailyForge" to Let's go button
• Added aria-current="step" to active step dot
• Added role="progressbar" and aria-valuenow/min/max to progress bar
• Replaced emoji icons (Sparkles, CheckCircle, Calendar, BarChart2, FileText, MousePointer, AlertTriangle, ClipboardList, Flame) with equivalent Lucide React components
• Replaced "Let's go! 🚀" with "Let's go!" (removed emoji)
• Replaced "← Back" and "Next →" text arrows with <ArrowLeft> and <ArrowRight> Lucide components

Impact it Made

• Onboarding modal now adapts to light and dark theme preferences
• All content is accessible via screen readers with proper labels
• Consistent visual language using the project's Lucide icon library
• Better rendering across different fonts and environments

Closes #1903
Closes #1905
Closes #1906

Note

Please assign this PR to the tmdeveloper007 account.