Handle mouse history navigation when Now is selected

[Ajeem-git]

Summary: Prevent a crash when mouse back/forward navigation reaches directory history while Now is selected.

What changed:

• Return early from MainWindow._dir_history() when selected_backup_id() is None.
• Treat mouse back/forward navigation as a no-op in the invalid Now state instead of raising NotImplementedError.

Why:
Directory history navigation is snapshot-based. When Now is selected there is no backup context to resolve the stored path against, so the safest behavior is to ignore the navigation request.

Verification:

• Traced the crash path from the issue traceback into qt/app.py.
• Verified the updated file compiles with python3.11 -m py_compile qt/app.py.

Note:
I could not fully reproduce the Qt GUI flow locally on macOS because the project expects Linux-oriented PyQt6 and dbus runtime dependencies.

[buhtz]

Hello Ajeem,

thank you for your contribution and taking time to improve Back In Time. I appreciate it.

On a quick initial review your PR looks good. I'll review it in detail later.

Please note that currently I have to put all PRs on hold, because the Back In Time code base is in a transition phase. I am rewriting the mounting subsystem from scratch (PR #2449). Since the components are not well isolated, this affects nearly every file. To avoid complex merge conflicts, all other PRs are temporarily on hold.

Best wishes,
Christian