feat(dashboard): redesign navigation and getting started

[stonexer]

What this does

Replaces the authenticated dashboard sidebar with the designer's top navigation and rebuilds Getting started using the supplied repository assets. Your own tools stays in primary navigation, Refer a friend stays visible at bottom right, and the latest Arena entry opens in a new tab.

Adds responsive light/dark styling, readable inactive navigation icons, token masking and clipboard failure feedback, keyboard-friendly menus, and navigation scroll resets. Activity, Team and Billing keep their existing internal layouts. Includes English design.md, Chinese design.cn.md, asset provenance, and the updated docs/context/interface/dashboard.md fragment with regenerated indexes. Visuals use static posters.

How it was tested

• uv run --with pytest-xdist pytest -n auto -q: 3614 passed, 6 skipped.
• uv run lint-imports: 14 contracts kept.
• Browser checks: desktop/mobile layout, light/dark selected and inactive icons, agent selection, copy success/failure, token reveal/masking, search, menus, and Back/Forward navigation. Rechecked desktop and 390px layout after integrating latest main.
• Added same-origin asset serving coverage and updated public Catalog navigation isolation assertions.

Checklist

• uv run --with pytest-xdist pytest -n auto -q passes locally
• Added or updated tests for the change (if it affects behavior)
• Updated the relevant docs/context/ fragment (if a subsystem changed)
• No secrets in the diff (keys, tokens, .env values)