fix(safe-mode): guard silent-mode confirm against rapid re-entry

[datlechin]

Summary

Both .silent branches in QueryExecutionCoordinator (dispatchStatements lines 68-84 and dispatchParameterizedStatements lines 141-153) now set the same isShowingSafeModePrompt re-entry flag the requiresConfirmation branch already used:

if level == .silent {
    guard !parent.isShowingSafeModePrompt else { return }
    parent.isShowingSafeModePrompt = true
    Task { [parent] in
        defer { parent.isShowingSafeModePrompt = false }
        ...
    }
}

The flag is set synchronously before the Task spawns, then cleared in a defer inside the closure.

Why this matters

This is bug R3 from the full-app audit. Holding Cmd+Return on a connection at safe-mode level .silent previously spawned multiple Tasks, each awaiting confirmDangerousQueryIfNeeded / confirmDangerousQueries. Two confirmation sheets could stack on top of each other, the user could confirm both, and the same dangerous query (e.g. DELETE FROM users WHERE 1=1) would run twice. The .requiresConfirmation branch already had this guard, so this PR brings the .silent branch in line.

Test plan

• Set safe-mode level .silent, hold Cmd+Return on a dangerous query, confirm only one confirmation sheet appears at a time.
• Confirm or cancel the sheet; the next Cmd+Return is processed normally afterwards.
• swiftlint --strict clean.