TUI: word-jump and line-jump cursor navigation in input field

[bug-ops]

Problem

Input field cursor navigation is limited to single-character movement with arrow keys. Home/End keys work but are not discoverable, and there is no word-level navigation.

Proposed solution

Add standard terminal cursor shortcuts in handle_insert_key() (crates/zeph-tui/src/app.rs):

Shortcut	Action
Alt+Left	Jump to previous word boundary
Alt+Right	Jump to next word boundary
Ctrl+A	Jump to start of line (same as Home)
Ctrl+E	Jump to end of line (same as End)
Alt+Backspace	Delete previous word

Implementation

1. Add prev_word_boundary(&self) -> usize and next_word_boundary(&self) -> usize helpers that scan self.input chars from cursor_position looking for word boundaries (transition between whitespace/punctuation and alphanumeric).

2. Handle KeyCode::Left with ALT modifier and KeyCode::Right with ALT modifier in the existing match block.

3. Map Ctrl+A / Ctrl+E to cursor_position = 0 / char_count() respectively.

4. Add Alt+Backspace to delete from cursor to previous word boundary.

5. Add tests for all new navigation and deletion shortcuts.

Notes

• On macOS, Cmd+Arrow keys are intercepted by the terminal emulator and do not reach the application, so Ctrl+A/E (emacs-style) is the standard approach.
• Word boundary detection should handle Unicode properly (use char::is_alphanumeric()).
• Existing Home/End handlers remain unchanged.