[Repo Assist] refactor: extract App::switch_mode to eliminate tab-switching duplication

[github-actions]

🤖 This issue was created by Repo Assist, an automated AI assistant.

Motivation

The 7-line mode-switch sequence was duplicated verbatim in three places in handler.rs:

app.mode = (new);
app.selected = 0;
app.selected_packages.clear();
app.detail = None;
app.loading = true;
app.set_status("Loading...");
app.refresh_view();

Duplicated in:

1. KeyCode::Tab | KeyCode::Right handler
2. KeyCode::BackTab | KeyCode::Left handler
3. handle_tab_click (mouse tab clicks)

Any future tab source (e.g. F1/F2/F3 shortcuts) would need to copy the same block again.

Change

Added App::switch_mode(mode: AppMode) to app.rs encapsulating the above sequence. All three call-sites now reduce to:

let new_mode = app.mode.cycle();
app.switch_mode(new_mode);
```

No behaviour change.

## Test Status

All 14 existing unit tests pass. `cargo clippy` — no warnings.

```
test result: ok. 14 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

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[shanselman]

Resolved in v0.3.0 release.