Iteration 4a: rebuild command with confirmation modal

Adds the explicit `rebuild` app-level command (ADR-0015 §7, §11)
and a modal UI infrastructure to host its confirmation dialog.
Typing `rebuild` emits Action::PrepareRebuild; the runtime reads
project.yaml + data/ to compute a summary ("3 tables and 47 rows
will be reconstructed; the existing playground.db will be
replaced") and posts AppEvent::RebuildPrepared, which opens the
modal. Y confirms, N/Esc cancels. While the modal is open,
normal input is gated.

The worker's do_rebuild_from_text now wipes existing user tables
and metadata before reloading from text, so it works on both
fresh and populated databases. Source text is plumbed through
rebuild_from_text so the explicit rebuild logs to history.log
while the silent on-load rebuild from Iteration 3 stays silent.

Modal infrastructure (App.modal field + key routing + centered
overlay rendering + word-wrap) is reused by Iteration 4b's save
/ save as / load / new flows.

Tests: 314 passing (268 lib + 9 + 5 + 6 new + 9 + 17),
0 failing, 0 skipped. Clippy clean.

tests/iteration3_rebuild.rs

@@ -69,7 +69,7 @@ fn rebuild_restores_schema_only_project() {
     )
     .unwrap();
     rt().block_on(async {
-        db.rebuild_from_text(project.path().to_path_buf())
+        db.rebuild_from_text(project.path().to_path_buf(), None)
             .await
             .expect("rebuild");
     });
@@ -137,7 +137,7 @@ fn rebuild_restores_rows_from_csv() {
     )
     .unwrap();
     rt().block_on(async {
-        db.rebuild_from_text(project.path().to_path_buf())
+        db.rebuild_from_text(project.path().to_path_buf(), None)
             .await
             .expect("rebuild");
     });
@@ -226,7 +226,7 @@ fn rebuild_restores_relationships_and_cascade_behaviour() {
     )
     .unwrap();
     rt().block_on(async {
-        db.rebuild_from_text(project.path().to_path_buf())
+        db.rebuild_from_text(project.path().to_path_buf(), None)
             .await
             .expect("rebuild");
     });
@@ -303,7 +303,7 @@ fn rebuild_reports_fatal_error_on_bad_csv_row() {
     .unwrap();
     let err = rt()
         .block_on(async {
-            db.rebuild_from_text(project.path().to_path_buf()).await
+            db.rebuild_from_text(project.path().to_path_buf(), None).await
         })
         .expect_err("must fail with row-level error");
     let msg = format!("{err}");
@@ -363,7 +363,7 @@ fn rebuild_preserves_created_at_from_yaml() {
     )
     .unwrap();
     rt().block_on(async {
-        db.rebuild_from_text(project.path().to_path_buf())
+        db.rebuild_from_text(project.path().to_path_buf(), None)
             .await
             .expect("rebuild");
     });