refactor(explore): rename misnamed "tag cloud" to "visual clusters"

The visual k-means cluster feature was confusingly named tag_cloud / tagCloud /
TagCloud across the whole stack, while the actual tag list is explore_tags — the
two were trivially easy to mix up (and did cause confusion). Rename the cluster
side to visual_cluster / visualCluster / VisualCluster everywhere: command
get_tag_cloud -> get_visual_clusters (+ lib.rs registration and the invoke
string), VisualClusterEntry, the store fields/actions/tokens, and the mock
backend. Old names are retired rather than reused, so any missed reference fails
loudly instead of silently resolving to the wrong concept.

The tags side keeps its accurate explore_tags naming, and the user-facing
"Tag Cloud" UI label is unchanged.

Also rename the SQLite tag_cloud_cache table -> visual_cluster_cache (the old
table is dropped during schema setup — it is a disposable cache already
invalidated by the clustering version bump) and the TagCloud.tsx component file
-> ExploreView.tsx, since it is the Explore container hosting both the cluster
and tag views.

src-tauri/src/vector.rs

@@ -268,7 +268,7 @@ pub fn get_embedding_revision(conn: &Connection) -> Result<String> {
 /// exact set of IDs, so it is membership-sensitive: adding, removing, or moving an
 /// image between folders changes it even when the count happens to stay the same.
 /// Used (together with the embedding revision, which catches an image being
-/// re-embedded in place) as the cheap tag-cloud cache key so a cache hit doesn't
+/// re-embedded in place) as the cheap visual-cluster cache key so a cache hit doesn't
 /// have to read and unpack hundreds of MB of embeddings just to validate freshness.
 pub fn embedding_ids_signature(conn: &Connection, folder_id: Option<i64>) -> Result<(i64, u64)> {
     use xxhash_rust::xxh3::Xxh3;