Add document statistics feature with background calculation

[romanstingler]

• Background document stats
  Added cached DocumentStatistics plus async calculation in src/main.rs so the statistics drawer never blocks the UI thread.
• State & messaging
  Extended App with document_stats, document_stats_dirty, document_stats_pending, document_stats_request_target, and document_stats_request_id, and introduced Message::DocumentStats to deliver background results.
• Visibility-based scheduling
  Updated App::update_tab(), the Message::TabChanged handler, and Message::ToggleContextPage so stats are recomputed only when the Document Statistics drawer is open for the active editor tab.
• UI rendering
  App::document_statistics() now renders cached values (or placeholders) without touching the editor buffer, avoiding main-thread work.

Method Details

• request_document_statistics_if_needed()(src/main.rs)
  Checks whether the statistics drawer is visible, whether a job is already running, and whether cached stats are still valid.
  If a refresh is required, it snapshots the active editor tab’s lines, assigns a new request ID, records the pending state, clears the dirty flag, and uses Task::perform + tokio::task::spawn_blocking to compute counts off the UI thread.
• calculate_document_statistics() (src/main.rs)
  Runs on the blocking thread, iterating through the provided lines to tally words, characters (with and without spaces), and lines. Returns a DocumentStatistics struct for the main thread to cache and display.

[mmstick]

This should be borrowed since it doesn't need ownership, and that sounds like a very costly vec.

[mmstick]

Seems like this should be in a DocumentStats struct?