Refactor insights handling and error encoding in presentation layer

- Updated insights analyzer tests to use Utc for date handling.
- Added timezone support for insights retrieval, allowing insights to be bucketed by the user's local time.
- Introduced `encode_domain_error` for better error handling in API responses.
- Modified various handlers to utilize the new error encoding function.
- Enhanced insights API to reject unknown time zones and return appropriate errors.
- Updated frontend components to handle insights data with timezone awareness.
- Improved localization for insights sample counts.

.env.example

@@ -47,6 +47,8 @@ ALLOW_REGISTRATION=true
 # SECURE_COOKIES=true
 # Handler-level logs come from the `presentation` crate; startup/wiring logs come
 # from `server`. Include both — `server=info` alone silently drops handler logs.
+# Crates not named here (adapters, etc.) still log warnings and errors unless
+# you set a bare level yourself (e.g. `info,presentation=debug`).
 # RUST_LOG=server=info,presentation=info,tower_http=info,worker=info
 
 # CORS (for SPA development only)