AD-701: Add web app shell

[alph-cmky]

Summary

• Adds a dense three-panel AgentDeck workspace shell scaffold.
• Adds top bar, left navigation sidebar, primary work area, and contextual inspector.
• Adds navigation entries for Chat, Agents, Workflows, Tasks, Runtimes, Audit, and Settings.
• Adds stable desktop panel widths and global CSS for the shell.
• Adds a Next-style src/app/page.tsx entry while keeping the current web package as a lightweight TypeScript package.
• Marks AD-701 complete in the development backlog.

Stack

• Stacked on AD-602: Add workflow validator #42 (feat/ad-602-workflow-validator).

Notes

• The current @agentdeck/web package does not yet include a real dev server/framework runtime, so this PR validates the shell through TypeScript/tests/build rather than a browser-served page.

Verification

• TDD red check: pnpm test apps/web/src/components/app-shell.test.ts initially failed because app-shell.js did not exist yet.
• pnpm test apps/web/src/components/app-shell.test.ts
• pnpm lint && pnpm format && pnpm typecheck && pnpm test && pnpm build

Closes #18