Web app skeleton and routing

Create Omni/Cal/Web.hs as a Wai application. Mount it at /cal in Omni/Web/Core.hs.

Routes:

• GET /cal → waterfall timeline (default view)
• GET /cal/agenda → plain agenda list
• GET /cal/week → rolling week grid
• GET /cal/search?q=... → search results
• GET /cal/events?from=DATE&to=DATE → htmx partial for infinite scroll

Query params for all views:

• calendars= comma-separated calendar names to show (persisted via cookie)
• from= start date (default: today)

Include a sidebar/header with:

• Calendar toggle checkboxes (which calendars are visible)
• View switcher links
• Search box

Use Omni.Web.Style design system (sharedShell, CSS custom properties, dark/light mode).