Add a dark theme toggle

[Aditya8369]

Description
No light/dark theme toggle exists in main site.

Plan
1.style.css:

• Add :root CSS vars for light theme (extract existing colors, e.g., --bg-gradient: linear-gradient(135deg, #fdf6f0, #f8e8ee, #e4f0f6);, --text-primary: #5c5470;, --accent: #ff86ba;, --glass-bg: rgba(255,255,255,0.6); etc. ~20 vars covering bg/text/buttons/shadows/gradients).
• Replace hardcoded colors with vars throughout (body, header, cards, footer, hover states, animations).
• Add body.dark block overriding vars to dark theme (e.g., --bg-gradient: linear-gradient(135deg, #1a0f2e, #2a1b3d, #3a254a);, dark glass: rgba(20,15,35,0.8);, light text).
• Update media queries/transitions for theme-aware smooth switches.

2.script.js:

• Add theme toggle function: toggleTheme() – checks localStorage/prefer-color-scheme, toggles document.body.classList.toggle('dark'), saves to localStorage.
• Init on load: initTheme() reads localStorage or system pref, applies class.
• Add toggle button listener (new ID #theme-toggle).
• Preserve existing nav/animations.

3.index.html (representative; apply to other main HTMLs via search/edit if needed):

• Add toggle button in .nav-profile: ☀️🌙.

4.auth.css:

• Similar to style.css: Add CSS vars (matching main), replace hardcoded colors, body.dark overrides (dark auth glassmorphism).

[Aditya8369]

@JiyaBatra please assign me this issue