feat(ui): relationships sidebar panel + schema data (#21, ADR-0046 DB2/DB4)

The left column now stacks a Tables panel over a Relationships panel.
Each relationship renders as three narrow lines — its name, then the
endpoints broken at the arrow (Customers.id -> / indented
Orders.customer_id) — ellipsized past the inner width. The panel is
content-sized within [5 rows ("(none)" when empty), half the column];
the Tables panel keeps the rest (>=3 rows). Phase C adds focus+scroll
for content beyond the cap (clipped for now).

Data path: a new worker Request::ReadAllRelationships +
Database::read_all_relationships returns full RelationshipSchema
records; the runtime posts them via a RelationshipsRefreshed event
alongside the schema-cache refresh, and the App holds them in a new
`relationships` field.

ADR deviation (recorded in ADR-0046 DB2 + index): DB2 specified this
data on SchemaCache; it lives on the App instead — SchemaCache is
walker/completion-facing and needs only relationship names (untouched),
while the full records are UI-only, so App is the cleaner home and it
avoids editing ~23 SchemaCache literals. No behavioural difference.

Tests: panel-height bounds, the three-line render, the empty "(none)"
case, a snapshot, read_all_relationships end-to-end (real DB via the
m:n junction), and the event->field handler.

tests/it/m2n.rs

@@ -416,3 +416,40 @@ fn pk_less_parent_is_refused() {
         assert!(format!("{err}").contains("no primary key"), "got: {err}");
     });
 }
+
+/// ADR-0046 DB2: the worker's `read_all_relationships` returns full
+/// schema records (name, parent/child tables + columns, actions) — the
+/// data source for the sidebar relationships panel. Exercised through
+/// the real worker thread after an m:n junction creates two of them.
+#[test]
+fn read_all_relationships_returns_the_junction_relationships() {
+    let (_project, db, _dir) = open();
+    rt().block_on(async {
+        serial_pk_table(&db, "Students").await;
+        serial_pk_table(&db, "Courses").await;
+        db.create_m2n_relationship("Students".to_string(), "Courses".to_string(), None, None)
+            .await
+            .expect("create m:n");
+
+        let rels = db
+            .read_all_relationships()
+            .await
+            .expect("read all relationships");
+        assert_eq!(
+            rels.len(),
+            2,
+            "the m:n junction creates two relationships: {rels:?}"
+        );
+        // Both have the junction (Students_Courses) as their child.
+        for r in &rels {
+            assert_eq!(r.child_table, "Students_Courses", "child is the junction: {r:?}");
+        }
+        // One points back to each parent.
+        let parents: std::collections::BTreeSet<&str> =
+            rels.iter().map(|r| r.parent_table.as_str()).collect();
+        assert!(
+            parents.contains("Students") && parents.contains("Courses"),
+            "one relationship per parent: {rels:?}"
+        );
+    });
+}