fix(ui): drive #mapError and #mapLoading background from theme tokens

[comnam90]

Spun out of code review on #103 (item #8).

Context

The `#mapError` overlay (shown when Leaflet fails to init) carries an inline RGBA background:

```html

\`\`\`

The hardcoded `rgba(10, 13, 18, 0.92)` bypasses the design tokens. If the user is on the light theme when the map fails to init, they see a dark overlay on an otherwise-light page. `#mapLoading` has the same inline style.

Proposed fix

Move the style to a CSS rule using `var(--bg)` with the right alpha, and provide an `html.light` override.

Acceptance

• Both overlays drive their background from theme tokens
• Visual check: trigger map init failure in light mode and confirm the overlay matches the page chrome