refactor(tui): split remaining app.rs into per-concern submodules

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

crates/tui/src/app/watcher.rs

@@ -0,0 +1,162 @@
+use log_viewer_core::io::file_reader::AppendStatus;
+use log_viewer_core::io::progressive_reader::{ReaderState, compute_line_visual_height};
+use log_viewer_core::watcher::file_watcher::FileEvent;
+
+use super::viewport_cache::gutter_width_for;
+use super::{App, AppLoadingState};
+
+impl App {
+    pub fn poll_file_watcher(&mut self) {
+        let events: Vec<FileEvent> = match &mut self.file_watcher {
+            Some(w) => std::iter::from_fn(|| w.try_recv()).collect(),
+            None => return,
+        };
+
+        for event in events {
+            match event {
+                FileEvent::Appended { new_size: _ } => {
+                    self.handle_file_appended();
+                }
+                FileEvent::Truncated { new_size: _ } => {
+                    self.handle_file_truncated();
+                }
+                FileEvent::Rotated { new_inode: _ } => {
+                    // Don't auto-switch; old content preserved.
+                    // User can reload manually if desired.
+                }
+                FileEvent::Removed => {
+                    self.loading_state = AppLoadingState::Error("File has been deleted".into());
+                }
+                FileEvent::WatcherError { message: _ } => {}
+            }
+        }
+    }
+
+    pub(super) fn handle_file_appended(&mut self) {
+        let rebased = self.rebase_offset_for_invalidate();
+        match &mut self.loading_state {
+            AppLoadingState::Ready { reader } => {
+                let old_reader_line_count = reader.line_count();
+                let status = reader.update_for_append();
+                let width = {
+                    let total = if self.content_width > 0 {
+                        self.content_width as usize
+                    } else {
+                        80
+                    };
+                    total.saturating_sub(gutter_width_for(reader.line_count(), false))
+                };
+                match status {
+                    Ok(AppendStatus::Appended(_new_lines)) => {
+                        let _ = reader.save_cache();
+
+                        let (old_line_count, can_extend) = {
+                            match &reader.state {
+                                ReaderState::Ready {
+                                    visual_height_index: Some(idx),
+                                    ..
+                                } => (idx.line_count(), idx.is_valid_for(self.json_format, width)),
+                                _ => (0, false),
+                            }
+                        };
+                        let new_line_count = reader.line_count();
+
+                        if can_extend && old_line_count == old_reader_line_count {
+                            if let ReaderState::Ready {
+                                visual_height_index: Some(index),
+                                reader: fr,
+                            } = &mut reader.state
+                            {
+                                if old_line_count > 0 {
+                                    let last_old_line_text =
+                                        fr.get_line(old_line_count - 1).unwrap_or("");
+                                    let new_h = compute_line_visual_height(
+                                        last_old_line_text,
+                                        width,
+                                        self.json_format,
+                                    );
+                                    index.replace_last_line_height(new_h);
+                                }
+                                let mut new_heights = Vec::with_capacity(
+                                    new_line_count.saturating_sub(old_line_count),
+                                );
+                                for i in old_line_count..new_line_count {
+                                    let line_text = fr.get_line(i).unwrap_or("");
+                                    new_heights.push(compute_line_visual_height(
+                                        line_text,
+                                        width,
+                                        self.json_format,
+                                    ));
+                                }
+                                index.extend_from_heights(&new_heights);
+                            }
+                        } else {
+                            let (new_offset, new_sub) = rebased;
+                            self.v_offset = new_offset;
+                            self.v_sub_offset = new_sub;
+                            self.cursor_sub_offset = 0;
+                            reader.invalidate_visual_height_index();
+                            reader.start_visual_height_rebuild(width, self.json_format);
+                        }
+
+                        self.viewport_cache.invalidate();
+                    }
+                    Ok(AppendStatus::Reloaded) => {
+                        let _ = reader.save_cache();
+                        let (new_offset, _new_sub) = rebased;
+                        reader.invalidate_visual_height_index();
+                        reader.start_visual_height_rebuild(width, self.json_format);
+                        self.cursor_line =
+                            self.cursor_line.min(self.total_lines().saturating_sub(1));
+                        self.v_offset = new_offset;
+                        self.v_sub_offset = 0;
+                        self.cursor_sub_offset = 0;
+                        self.viewport_cache.invalidate();
+                        self.clamp_v_offset();
+                    }
+                    Ok(AppendStatus::Unchanged) | Err(_) => {}
+                }
+            }
+            AppLoadingState::Loading { .. } => {
+                self.reload_after_loading = true;
+            }
+            _ => {}
+        }
+    }
+
+    pub(super) fn reload_ready_reader(&mut self) {
+        let (new_offset, _new_sub) = self.rebase_offset_for_invalidate();
+        if let AppLoadingState::Ready { reader } = &mut self.loading_state {
+            let _ = reader.reload();
+            let width = {
+                let total = if self.content_width > 0 {
+                    self.content_width as usize
+                } else {
+                    80
+                };
+                total.saturating_sub(gutter_width_for(reader.line_count(), false))
+            };
+            let _ = reader.save_cache();
+            reader.invalidate_visual_height_index();
+            reader.start_visual_height_rebuild(width, self.json_format);
+            self.cursor_line = self.cursor_line.min(self.total_lines().saturating_sub(1));
+            self.v_offset = new_offset;
+            self.v_sub_offset = 0;
+            self.clamp_cursor_sub_offset_no_vhi();
+            self.viewport_cache.invalidate();
+            self.clamp_v_offset();
+        }
+    }
+
+    pub(super) fn handle_file_truncated(&mut self) {
+        match &mut self.loading_state {
+            AppLoadingState::Ready { .. } => {
+                self.reload_ready_reader();
+            }
+            AppLoadingState::Loading { .. } => {
+                self.reload_after_loading = true;
+            }
+            _ => {}
+        }
+    }
+}