cross-cutting: setTheme(dark|light|auto) with graceful degradation

[productdevbook]

Runtime dark mode support varies wildly.

Evidence

• `[CW-2692] Unread message not working in dark mode chatwoot/chatwoot#8199` [CW-2692] Unread message not working in dark mode chatwoot/chatwoot#8199
• `feat: Exposing setWidgetColor function through javascript chatwoot/chatwoot#13039` (runtime setWidgetColor) feat: Exposing setWidgetColor function through javascript chatwoot/chatwoot#13039
• Intercom: auto-detects `prefers-color-scheme`
• Beacon: custom CSS only

Tasks

• `setTheme({ mode: "light" | "dark" | "auto", color?: string })`
• Per provider: runtime API where available, reinit with new config otherwise, no-op as last resort
• Listen to `matchMedia('(prefers-color-scheme: dark)')` when `mode: "auto"`

[productdevbook]

Implemented in this commit