Refactor todo view sorting to explicit mode enum

[multiplex55]

Motivation

• Replace the ad-hoc boolean sort_by_priority with an explicit mode to make sorting behavior clearer and extendable.
• Make sorting deterministic by adding stable tie-breakers so the displayed order is predictable across runs.
• Replace the checkbox UI with an explicit selector so users can choose sort Priority or Name while keeping the existing filter behavior.

Description

• Introduce enum TodoSortMode { Priority, Name } and replace TodoViewDialog.sort_by_priority: bool with sort_mode: TodoSortMode, defaulting to Priority in open() and open_edit().
• Add a pure helper fn sorted_todo_indices(entries, filter, sort_mode) -> Vec<usize> that centralizes filtering and index-based sorting and preserves index indirection for edits/removals.
• Implement deterministic tie-breakers: in Priority mode sort by priority (desc) then text.to_lowercase() (asc) then original index; in Name mode sort by text.to_lowercase() (asc) then priority (desc) then original index.
• Replace the checkbox with an egui::ComboBox offering Priority and Name, and add unit tests in the existing #[cfg(test)] mod tests that exercise priority ordering, name ordering, stability/tie-breaks, and filter+sort interaction.

Testing

• Ran cargo fmt --all which completed successfully.
• Ran cargo test src::gui::todo_view_dialog -- --nocapture but the build/test run failed in this environment due to a system dependency error from alsa-sys (pkg-config / missing alsa.pc), so the new unit tests could not be executed here.

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