docs: document v0.18.0 API additions (#108)

Cover the new public API shipping in v0.18.0 across the reference docs,
llm.md, and the CLAUDE.md option pointers:

- Border title alignment (WithBorderTitleAlign), independent title and
  focus border styling (WithBorderTitleStyle, WithFocusBorderStyle)
- App render hooks (WithPreRenderHook, WithPostRenderHook) and
  WithManualCursor
- Real-cursor protocol (WithCursorSource/SetCursorSource, CursorReporter)
  and the per-widget virtual-cursor opt-in
- TextArea/Input programmatic editing (CursorPos, SetCursorPos, InsertText)
  and WithTextAreaOnChange
- Element runtime resize setters (SetWidth, SetHeight)