fix: float Git branch picker above workspace content

Render the searchable branch picker in the shared floating portal so opening it no longer pushes down changes or commit history. Clamp it to the viewport and retain search focus, outside dismissal, and Escape focus restoration.

Update existing migration fixtures for activity history and supervision schema changes. Focused suites passed: 149 storage tests, 41 UI tests, and the Electron overlay test; typecheck and lint passed. The full suite did not complete cleanly and reported failures outside this change.

Release note: The Git branch picker now opens over the workspace without displacing the file list or commit history.

src/main/magic-notes/magic-note-storage.test.ts

@@ -218,7 +218,10 @@ describe('Magic note SQLite and filesystem storage', () => {
       const entry = note.entries[0]!
       sql.prepare('UPDATE magic_note_entries SET content_json = ? WHERE id = ?').run(JSON.stringify(entry.content), entry.id)
     }
-    sql.exec('PRAGMA user_version = 37')
+    sql.exec(`ALTER TABLE supervision_results DROP COLUMN graph_snapshot_json;
+      DROP TABLE activity_history_records;
+      ALTER TABLE activity_history RENAME COLUMN record_order_json TO records_json;
+      PRAGMA user_version = 37`)
     rmSync(join(directory, 'notes'), { recursive: true })
     const progress: number[] = []
     upgradeAssistantStorage(path, (value) => progress.push(value.processed))