Unify task web navbar to double-bar pattern matching news/files

The /news and /files pages use a clean double-bar navigation: top navbar + sub-navigation bar below it. The /tasks pages use a single collapsible hamburger sidebar that's less usable.

Unify the task web UI to use the same double-bar pattern:

• Top bar: "omni" brand link (to /), then top-level nav: tasks, news, files, stats
• Second bar (sub-nav): context-sensitive links for the current section. For tasks: ready, blocked, needs help, all, epics, kb

Implementation:

• Update navbar in Omni/Task/Web/Components.hs to match the newsreader pattern (brand + top links in one bar, section links in a second bar)
• Remove the hamburger toggle and dropdown menus
• Update CSS in Omni/Task/Web/Style.hs or Omni/Web/Style.hs to style the sub-nav bar
• The newsreader uses class nr-subnav; create a shared class like subnav or adapt the existing navbar styles

Key files: Omni/Task/Web/Components.hs (navbar function), Omni/Task/Web/Style.hs, Omni/Web/Style.hs Namespace: Omni/Task.hs