Stage 3: Plan editor with Steps, Supersets and reordering

- Plan model with immutable edits: add with Mode defaults, move, remove,
  Superset with next, take a member out, ungroup; Stretching length
  estimate from holds, sides, rounds and Transitions
- drift database (plans table, items as JSON, ADR 0005) and
  PlanRepository; Plans list per Mode with rename/duplicate/delete
- Editor: drag handles, step editor sheet (sets, reps or time, optional
  kg; hold, both sides, rounds; Superset sets and rest between),
  pause between holds for Stretching Plans
- Exercise picker reusing the Library browser; Add to plan from the
  Exercise detail page
- Theme: Mode-styled snackbar, FAB, menus, segmented buttons, switches

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/support/test_database.dart

@@ -0,0 +1,21 @@
+import 'dart:ffi';
+import 'dart:io';
+
+import 'package:drift/drift.dart';
+import 'package:drift/native.dart';
+import 'package:sqlite3/open.dart';
+import 'package:zenxercise/src/database/database.dart';
+
+/// An in-memory database for tests. On Windows hosts the sqlite3 library comes
+/// from ZENXERCISE_SQLITE_DLL if set, otherwise from the system's winsqlite3.dll.
+AppDatabase testDatabase() {
+  if (Platform.isWindows) {
+    open.overrideFor(OperatingSystem.windows, () {
+      final custom = Platform.environment['ZENXERCISE_SQLITE_DLL'];
+      return DynamicLibrary.open(custom ?? 'winsqlite3.dll');
+    });
+  }
+  driftRuntimeOptions.dontWarnAboutMultipleDatabases = true;
+  // Synchronous stream teardown keeps widget tests from hanging on pending timers.
+  return AppDatabase(DatabaseConnection(NativeDatabase.memory(), closeStreamsSynchronously: true));
+}