fix(canvas): dark theme a11y — settings buttons, input fields, ReactFlow colorMode, zinc-400 contrast, aria-labels
Resolves low-contrast text and theming issues in the settings panel and
canvas overlays when running in dark mode:
- settings-panel.css: input fields (#d4d4d8 text), settings-button--active
(#1e3a8a bg for better contrast against #3b82f6 accent)
- SearchDialog: placeholder-zinc-400, kbd hints, tier badge, footer counts,
empty-state text — all lifted from zinc-600 → zinc-400
- ConversationTraceModal: timestamp, arrow separators, truncation ellipsis
— lifted from zinc-600 → zinc-400
- CommunicationOverlay: arrow separator, age label, duration — zinc-600 → zinc-400
- TemplatePalette: dynamic aria-label on toggle button
("Open/Close template palette") for screen-reader clarity
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>