Dark mode

Adds light / dark / system theming. The layout previously force-locked light; now there's a real theme system with a Settings toggle.

What's added

• Theme bootstrap runs before paint (no flash) — resolves the stored preference (light | dark | system, default system) against prefers-color-scheme. A Settings → Appearance segmented control persists the choice and applies it live, and keeps system in sync if the OS theme flips while the page is open.
• Dark theme via a scoped .dark CSS remap. The views use a small, consistent utility palette (bg-white cards, zinc borders, neutral text), so one place remaps them — surfaces, borders, text, hover states, and inverted primary buttons — instead of adding dark: variants to ~380 call sites. Unlayered <head> styles beat crosswind's layered utilities.
• Full coverage: the shell (sidebar/workspace, targeted by data-attribute), the analytics chart's hardcoded SVG palette (so it's not a light island), the timeline feed and toast stylesheets (their own classes, not utilities), and the login page (separate layout — self-contained bootstrap + overrides).

Verification (browser)

Screenshotted in dark across composer, queue (list and calendar), analytics (chart blends in), settings (toggle), accounts, timeline (feed), and login — all readable and consistent. Light mode unchanged (the .dark-scoped rules only activate with the class — confirmed no regression). tsc exit 0; 36 tests; pickier clean.

Off the optional MVP list.