Add outline popup window for folder context menu

Implement the outline file popup window that opens when a user
right-clicks on a folder in the file list (e.g., Act 1 in 06-First Draft)
and selects 'Outline' from the context menu. The window displays the
corresponding outline file (e.g., 04-Outline/scene_breakdown/act_01.md)
in a read-only view.

Changes:
- Add outline_popup_window() function in file_list.rs
- Add window rendering call in mod.rs update loop
- Popup reads file content on open and displays it in scrollable view

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/app/mod.rs

@@ -312,6 +312,9 @@ pub struct App {
     show_manuscript_settings: bool,
     /// Whether the Outline window is open.
     show_outline: bool,
+    /// Whether to open an outline file from a folder context menu.
+    /// Contains the workspace-relative path of the outline file to open.
+    open_outline_file: Option<String>,
     /// Whether the diff window is open, and what it is showing.
     show_diff: bool,
     diff_text: String,
@@ -434,6 +437,7 @@ impl App {
             show_characters: false,
             show_manuscript_settings: false,
             show_outline: false,
+            open_outline_file: None,
             show_diff: false,
             diff_text: String::new(),
             diff_title: String::new(),
@@ -589,6 +593,10 @@ impl eframe::App for App {
             self.diff_window(ctx);
         }
 
+        if self.open_outline_file.is_some() {
+            self.outline_popup_window(ctx);
+        }
+
         if self.show_cheatsheet {
             crate::help::cheatsheet_window(ctx, &mut self.show_cheatsheet);
         }