Rename `--shinychat-thinking-*` CSS custom properties to the `--shiny-chat-*` prefix

The thinking display exposes three author-settable CSS custom properties under a `--shinychat-*` prefix, which is inconsistent with the rest of shinychat's public CSS API (`--shiny-chat-*`, e.g. `--shiny-chat-command-chip-color`, `--shiny-tool-card-*`, and the newly added `--shiny-chat-activity-opacity`). The inconsistent names, all defined on `.shinychat-thinking` in `js/src/chat/chat.scss`: - `--shinychat-thinking-header-color` - `--shinychat-thinking-border-color` - `--shinychat-thinking-content-color` Proposed rename: - `--shinychat-thinking-header-color` → `--shiny-chat-thinking-header-color` - `--shinychat-thinking-border-color` → `--shiny-chat-thinking-border-color` - `--shinychat-thinking-content-color` → `--shiny-chat-thinking-content-color` ## Notes - These three properties are the only `--shinychat-*` (no hyphen) custom properties in the codebase; everything else already uses `--shiny-chat-*`. - They are not referenced in any package documentation (R or Python), so the public exposure is real but unadvertised. - This is a breaking change for anyone who has set the old names. Rebuild and redistribute to both packages after the rename (`npm run build`, `make py-update-dist r-update-dist`). - Surfaced while adding `--shiny-chat-activity-opacity` (the shared tool/thinking activity-dim token), which deliberately followed the `--shiny-chat-*` convention. ## Related The same `.shinychat-*` vs `.shiny-chat-*` inconsistency exists in the **class** names, tracked separately in #286 (higher blast radius, since those are rendered DOM classes).