docs: premium README visuals (hero, architecture, features, quickstart)

[OnlyTerp]

Summary

Adds a set of premium, on-brand graphics to the README to make the project look polished and high-quality. No code or behavior changes — docs/assets only; the offline self-test (test_proxy.py) still passes.

Four 2× PNGs under assets/brand/ (rendered from HTML/CSS at deviceScaleFactor: 2), matching the ultracode app icon's indigo→violet→purple→fuchsia palette on a near-black background, with Inter + JetBrains Mono type:

• hero.png — header banner: logo tile, gradient wordmark, tagline, feature pills.
• features.png — 4-card highlights row (one icon/any model · stdlib-only · tools translated · Claude untouched).
• architecture.png — replaces the ASCII flow diagram under a new ## How it works heading: Claude Code /model → proxy.py (UltraCode envelope) → backends, with ANTHROPIC_BASE_URL / GET /v1/models connectors.
• quickstart.png — 3-step strip (get code → pick models → launch & /model).

README changes:

• Header swapped for centered hero image + recolored badge row (added a platforms badge).
• ASCII Claude Code ──▶ proxy.py ──▶ model block replaced by architecture.png.
• Quick start split into Windows / macOS / Linux / WSL subsections with the quickstart graphic on top.
• assets/brand/README.md documents the assets, palette, and how to regenerate them.

Each image is referenced via centered <p align="center"><img …></p> with descriptive alt text.

Link to Devin session: https://app.devin.ai/sessions/92667378068c424b84282a40b0290647
Requested by: @OnlyTerp

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✅ Devin Review: No Issues Found

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