feat(web): add kiosk mode for simplified UI

[cboecking]

Summary

Add kiosk mode to the web UI that provides a simplified user experience by hiding navigation elements.

URL Parameters

Parameter	Description
?kiosk=true	Hides sidebar and header
?dir=/path	Auto-opens project and navigates to new session

Use Cases

• Embedded deployments
• Simplified onboarding experiences
• Kiosk terminals
• Single-project focused workflows

Example URLs

# Basic kiosk mode
http://localhost:4096/?kiosk=true

# Kiosk with auto-project
http://localhost:4096/?kiosk=true&dir=/home/user/my-project

Changes

• packages/app/src/context/layout.tsx - Added kiosk.enabled() and kiosk.dir() state
• packages/app/src/pages/layout.tsx - Conditional sidebar rendering
• packages/app/src/components/session/session-header.tsx - Conditional header rendering
• packages/app/src/pages/home.tsx - Auto-redirect logic for dir parameter
• docs/KIOSK_MODE.md - Feature documentation

Test Plan

• [x] Verify ?kiosk=true hides sidebar (desktop)
• [x] Verify ?kiosk=true hides sidebar (mobile)
• [x] Verify ?kiosk=true hides header
• [x] Verify ?dir=/path auto-redirects to new session
• [x] Verify normal mode unaffected without parameters
• [x] Type checking passes