Add status bar beneath chat input with send/stop button and context display

Restructure ChatInput into two stacked bars:
- Top bar: auto-resizing textarea + fixed-width send/stop button that
  morphs in place (no layout shift) across idle/generating states.
- Bottom bar: agent status icon, context-window fill bar, and compact
  token count + percent (inert bar when model max is unknown).

Wire contextLimit prop from App.svelte into ChatInput, reusing the
shared computeContextUsage helper so it agrees with the sidebar.

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