feat(usage): refactor dashboard frontend with shadcn/ui components, skeleton states, and dark-theme charts

.impeccable.md

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+## Design Context
+
+### Users
+
+Developers and power users who manage multiple AI accounts (Google Gemini, Claude) through a desktop Electron app. They use IDEs like Cursor, OpenCode, and Windsurf. They need to monitor token usage across accounts in real-time to avoid quota exhaustion.
+
+### Brand Personality
+
+- **Professional**: Clean, precise, no clutter
+- **Technical**: Data-dense but readable, developer-friendly
+- **Efficient**: Information at a glance, minimal clicks
+- 3 words: "precise", "technical", "focused"
+
+### Aesthetic Direction
+
+- Dark mode primary (this is a developer tool used for long sessions)
+- Deep navy/slate backgrounds, not pure black
+- Sharp accent colors for data visualization (subtle blues, teals, amber for warnings)
+- High information density without visual noise
+- Monospace for numbers/data, clean sans-serif for UI chrome
+- No gradients on text, no glassmorphism, no rounded-everything
+
+### Design Principles
+
+1. Data first — charts and numbers are the heroes
+2. Every pixel earns its place — no decorative elements
+3. Density over whitespace — developers want to see everything
+4. Motion only for state changes — no gratuitous animations
+5. Consistent with shadcn/ui + Tailwind system already in place