Restyle UI with brutalist theme

[rishabhcli]

Summary

• refresh the application palette with a brutalist-inspired color system and textured background grid
• restyle sidebar, drop zone, buttons, and file list with chunky outlines while retaining existing animations
• update modern component gradients/badges and document completed brutalist to-dos alongside the new theme color

Testing

• npm test -- --runInBand (fails: option unsupported in vitest)
• npm test (runs in watch mode and was interrupted after partial execution)

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Codex Task