Refine Agent identity controls

[wowalxa2-tech]

Description

Refine the Agent launcher and header identity controls to match the light Host design system.

Changes

• Use round pale-blue Host accents for the Agent launcher and Agent title mark.
• Reduce the launcher to 48px while keeping the title mark at 40px.
• Use the existing Host ink color for the Agent glyph and remove the black identity blocks.
• Add frontend regression coverage and synchronize the frontend color requirements.

Scope

Visual presentation only. No Agent routing, message handling, Plugin/Resident behavior, database, or API logic changes.

Verification

• npm run build:frontend
• npm run check:frontend
• npm run test:frontend — 95 passed
• .venv/bin/python scripts/check-t8-docs.py — 26 artifacts
• git diff --check