Feature request: filter favourites within selected sources

[hyungyunlim]

Feature request

Add a general filter_favorites / filter_favourites option that limits the kiosk to favourite assets while still respecting the currently selected sources.

Use case

I want to show only favourite assets from a selected source, for example:

• favourite assets within selected albums
• favourite assets for selected people
• favourite assets with selected tags
• favourite assets within rating/date/random source filters

The current album=favorites style behavior works as a global favourites source, but it does not cover the case where favourites should be used as an additional filter on top of another source selection.

Proposed behavior

• Keep the existing album=favorites special keyword as global favourites.
• Add a separate favourite filter option that can be combined with existing source filters.
• Apply the favourite filter through the shared search paths for albums, people, tags, rating, random assets, and date ranges.
• Expose the option in the config schema, example config, and URL builder.

Rationale

Immich search request DTOs already support isFavorite alongside source filters such as albumIds, personIds, tagIds, and rating. Treating favourites as a general filter should keep filtering server-side where possible and avoid making favourites an album-specific workaround.

Related context

This was originally explored in PR #786, but it should be tracked as an issue/feature request first rather than as an unsolicited PR.