v0.18.0

Doorman, when you happen to run it (#37)

Waikiki now notices the sibling Doorman
app and routes text-to-speech through its Voicebox voices instead of the
browser's robotic speechSynthesis. Strictly optional in both directions:
nothing is installed or launched on your behalf, and if Doorman isn't running
Waikiki says nothing and uses its own voice. Settings shows what it found.

Using Doorman's agents for chat-with-page needs an endpoint that doesn't exist
there yet — tracked as VerinFast/doorman#78.

Fixes

• The lead section's edit and speaker controls were invisible. They hide at
  opacity:0 and are revealed by hovering a heading — but the lead section (the
  text before the first heading) has no heading, so nothing could ever reveal
  them. They now sit faintly visible at rest.
• Speaker buttons were black on black. .tts-btn set no color, so it
  inherited the browser default black: readable on the light theme, invisible on
  dark and sepia. This affected every heading's speaker, not just the lead's.
• Right-click menus closed on their own opening gesture. A ctrl-click is a
  left click, so the click that opens the menu also dismissed it — the menu only
  appeared while the button was held. Dismissal now waits for a genuinely new
  gesture.

tests/test_theme_contrast.py guards the invisible-control class of bug by
reading the stylesheet, since asserting an element is in the DOM proves nothing
about whether it can be seen.