feat: redesign UI — sidebar cards, welcome screen, refined design tokens

[Copilot]

Refreshes the ZenMark UI with a more polished visual design across the sidebar and welcome screen, without changing any application behavior or data layer.

Sidebar

• Replaced flat note list with card-style rows showing title, relative timestamp, and a markdown-stripped content preview (72 chars)
• Branded header: gradient ✦ icon + "ZenMark" text + live note count badge
• Pin/delete actions are hover-revealed to reduce visual noise
• Search bar gains a focus ring and inline clear (×) button using icon-btn
• Template dropdown uses glassmorphism + CSS class-based hover (removes inline onMouseEnter/Leave style mutations)

Welcome Screen

• Tagline changed from "Welcome to ZenMark" → "Write. Think. Zen."
• Added ✦ Markdown Notes pill badge above title
• Feature cards use icon background wraps (.feature-icon-wrap) instead of raw icon color
• Staggered fadeInUp animations on hero, actions, and feature grid

Design tokens (index.css)

• Darker, higher-contrast dark theme (#0d1117 / #13191f) aligned with GitHub's palette
• New tokens: --note-card-radius, --transition-fast, --transition-normal
• New classes: .template-item, .note-card, .search-bar, .sidebar-brand, .welcome-badge

Screenshots

Dark theme — Welcome Screen

Dark theme — Editor with note cards

Light theme — Editor

Original prompt

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Created from VS Code.

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