## Why

Theme choice is part of a user's identity and comfort on the site; returning visitors should land in the theme they previously selected. We also need measurement to understand whether the theme switcher is being used and which themes are preferred.

## What Changes

- Persist the user's selected theme across visits so returning users see the last-selected theme immediately.
- Add Umami tracking for theme selection changes so theme switch usage can be measured and segmented.
- Improve robustness of persistence by supporting either localStorage or a client-side cookie (cookie fallback when localStorage is unavailable).

## Capabilities

### New Capabilities
- (none)

### Modified Capabilities
- `site-theming`: Extend theme persistence requirements to explicitly cover returning visits and define acceptable client-side persistence mechanisms / fallback behavior.
- `analytics-umami`: Add a custom event emitted from client-side code for theme selection changes (using Umami's JS API when needed).
- `interaction-tracking-taxonomy`: Define the theme selection event name and required event properties (at minimum `target_id` and `placement`, plus theme metadata).

## Impact

- Frontend: update theme switcher behavior in `site/src/layouts/BaseLayout.astro` (persistence/fallback and event emission).
- Analytics: new Umami event(s) added; dashboards/filters can segment by selected theme and placement.
- Specs: update the modified capabilities above to reflect the new requirements.