* fix(room-list): instantiate message previewers lazily
Removing the unused SettingsStore import from MessagePreviewStore (when
the
feature_new_room_list flag is dropped) changed module load order and
exposed a latent circular dependency: ReactionEventPreview imports
MessagePreviewStore, which eagerly did `new ReactionEventPreview()` at
module-eval — so importing ReactionEventPreview first (as its unit test
does) hit "ReactionEventPreview is not a constructor".
Construct the previewers lazily on first use (cached) instead of at
module
load, so nothing dereferences a mid-evaluation module. Fixes
ReactionEventPreview-test.
* refactor: pass MessagePreviewStore to ReactEventPreview
* refactor: move `MessagePreviewStore` and previews into its own directory
The `MessagePreviewStore` is used widly and not only by the room list.
Moving to its own folder to be able to remove old room list later with
less friction
* test: add more tests