fix(web): clarify agent dock and layout controls

[gbasin]

What changed

• collapse the desktop navigation into a persistent 52px rail instead of removing it
• keep the collapsed agent dock launcher anchored at the top
• make the agent header the single split/focus control and preserve navigation state independently
• remove duplicate persistent New agent buttons; the composer audience toggle is the canonical creation path
• suppress the mobile agent dock beneath the navigation drawer
• prevent pointer-driven navigation collapse from leaving its focus tooltip visible
• use assertive announcements only for error toasts and improve strict-filter empty-state copy
• update configured-agent E2E flows to enter through composer agent mode

Why

The recent agent-shell update conflated focus mode with hiding navigation, left the collapsed agent launcher vertically centered, and exposed several redundant agent-creation actions. On mobile, the launcher could also float above the navigation scrim. The result made layout controls feel unpredictable and the shell unnecessarily busy.

User impact

Navigation now remains useful when collapsed, focus mode affects only the agent pane, the agent dock has a stable top anchor, and agent creation has one consistent visible entry point. Mobile layering and accessibility announcements are also clearer.

Validation

• pnpm check
• pnpm lint
• pnpm --filter @atrium/web typecheck
• pnpm --filter @atrium/web build:ci
• targeted Chromium proof and configured-agent flows: 6/6 passed after rebasing onto current master
• full Playwright run: 119 application scenarios passed; the sole failure was an over-broad new proof assertion that counted an unrelated timestamp tooltip. After scoping it to the navigation tooltip, the proof spec passed 2/2.