Assert list_state is selected in Detail movement methods

[ooloth]

Adds precondition assertions to all four Detail-arm movement methods (move_up, move_down, move_page_up, move_page_down) so that an unselected list_state in the Detail screen panics immediately rather than silently computing movement from index 0.

Since apply_enter_action always calls ds.select(Some(0)) when entering the detail screen, an unselected state here is a programmer error, not a recoverable case.

Closes #48

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Generated by Claude Code

[Copilot]

Pull request overview

This PR adds precondition assertions to App’s Screen::Detail movement methods to fail fast when DetailView.list_state is unexpectedly unselected, instead of silently defaulting to index 0.

Changes:

• Added assert!(view.list_state.selected().is_some(), ...) to move_up, move_down, move_page_up, and move_page_down in the Screen::Detail arms.
• Intended to treat an unselected Detail list state as a programmer error (per issue #48).

Comments suppressed due to low confidence (2)

ui/tui/src/state/mod.rs:82

• Same issue as in move_up: apply_enter_action leaves DetailView.list_state unselected when item_count == 0 (ui/tui/src/state/update.rs:297-306). With the new unconditional assert, move_down will panic instead of being a no-op for an empty detail list. Consider asserting only when len > 0, or ensure the detail screen always selects an index.

                assert!(
                    view.list_state.selected().is_some(),
                    "list_state must be selected in Detail screen"
                );
                let sel = view.list_state.selected().unwrap_or(0);

ui/tui/src/state/mod.rs:120

• This assertion will also fire for an empty detail list, but apply_enter_action can construct Screen::Detail with list_state left unselected when item_count == 0 (ui/tui/src/state/update.rs:297-306). If empty groups are allowed, move_page_up should probably no-op rather than panic; otherwise make the invariant true by always selecting when entering Detail.

                assert!(
                    view.list_state.selected().is_some(),
                    "list_state must be selected in Detail screen"
                );
                let sel = view.list_state.selected().unwrap_or(0);
                view.list_state.select(Some(sel.saturating_sub(PAGE)));

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