Add timestamp insertion in notes editor context menu

[multiplex55]

Motivation

• Provide a quick way for users to insert timestamps into note content using the same stable timestamp format used elsewhere in the project.
• Ensure timestamp insertion is Unicode-safe and respects the current caret position in the multiline editor.

Description

• Add format_note_timestamp(dt: DateTime<Local>) and format_note_timestamp_now() that format timestamps as %Y-%m-%d %H:%M:%S to match existing conventions.
• Add insert_at_char_boundary(text, idx, insert) which inserts by character index, clamps out-of-range indices to the end, and avoids splitting UTF-8 code units.
• Replace TextEdit::multiline(...).show(...) output usage to capture cursor_range and obtain the caret character index, and attach a context menu item with the exact label Insert timestamp that inserts the formatted timestamp at the caret (or appends as a fallback) and closes the menu.
• Preserve existing Enter key consumption logic and the Save/Cancel flow in the edit-mode block.
• Add unit tests for middle/start/end insertion, out-of-range fallback, Unicode-safe boundaries, and deterministic timestamp formatting using a fixed datetime.

Testing

• Ran cargo fmt -- src/gui/notes_dialog.rs which completed successfully.
• Ran cargo test --lib notes_dialog -- --nocapture, which could not complete in this environment due to a missing system dependency (alsa pkg-config/alsa.pc) required by a transitive crate; the failure is environmental and not from the newly added unit tests or logic.
• Unit tests were added under src/gui/notes_dialog.rs in the #[cfg(test)] module covering the insertion and timestamp formatting cases.

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Codex Task