Refactor tests to avoid mutating TUI context directly

[panarch]

Summary

Several notebook view snapshot tests currently poke into App::context_mut() to flip internal state (e.g. setting last_log, mutating tabs, forcing vim_keymap). This bypasses real user flows and makes the tests fragile.

Examples

• tui/tests/notebook_views.rs manually edits context.notebook.editors, pushes synthetic tabs, and sets breadcrumb vectors directly.
• The same file toggles context.vim_keymap to display overlays instead of driving the key sequences that would normally open them.

Impact

• Tests couple snapshots to internal implementation details instead of public behaviour.
• It is hard to refactor the TUI context because tests rely on private state.
• App::context_mut remains in tui/app.rs purely to support these tests.

Proposal

1. Introduce helper functions or actions that exercise the notebook flows through real transitions (e.g. dedicated commands to show Vim keymaps, helpers for log/saving indicators).
2. Refactor tests to use those helpers or existing input sequences instead of reaching into context.
3. Remove App::context_mut entirely once no tests depend on it.

Acceptance Criteria

• Notebook view snapshot tests operate via user-level inputs or dedicated helpers; no direct context mutation remains.
• App::context_mut is no longer needed and is removed from tui/app.rs.
• Existing coverage stays unchanged after the refactor.