Add editor text zoom slider

A Zoom slider (with Reset) above the editor scales the monospace font
from -50% to +200%, defaulting to 0%. The editor keeps infinite desired
width inside a vertical-only scroll area, so text wraps to the pane at
any zoom with no horizontal scrolling. The zoom level persists in config.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -64,7 +64,9 @@ Terminal=false
    After that, the **⟳ Sync (git)** button commits all changes, `pull --rebase`s,
    and pushes.
 3. Create files with **＋ New**, edit on the right, `Ctrl+S` (or the Save button)
-   to write to disk. Drag the `⠿` handles to reorder.
+   to write to disk. Drag the `⠿` handles to reorder. Use the **Zoom** slider
+   above the editor to scale the editor text (0% = default; **Reset** returns to
+   it). Text always wraps to the pane width, so there is no horizontal scrolling.
 4. Set the **Chapter title** for the open file if you want to override the
    auto-derived one (the field shows the automatic title as a hint). Leaving it
    blank falls back to a `# Title:` line in the header, and if there is none, to

src/app.rs

@@ -664,6 +664,27 @@ impl App {
                             self.dirty = true; // ensure save runs
                             self.save_current();
                         }
+                        ui.separator();
+                        ui.label("Zoom:");
+                        if ui
+                            .add(
+                                egui::Slider::new(&mut self.config.editor_zoom, -50.0..=200.0)
+                                    .suffix("%")
+                                    .step_by(5.0),
+                            )
+                            .on_hover_text("Editor text size (0% = default)")
+                            .changed()
+                        {
+                            self.config.save();
+                        }
+                        if ui
+                            .button("Reset")
+                            .on_hover_text("Reset zoom to 0%")
+                            .clicked()
+                        {
+                            self.config.editor_zoom = 0.0;
+                            self.config.save();
+                        }
                     });
                     ui.horizontal(|ui| {
                         ui.label("Chapter title:");
@@ -716,6 +737,17 @@ impl App {
     }
 
     fn editor(&mut self, ui: &mut egui::Ui) {
+        // Base monospace size scaled by the zoom percentage. `desired_width` stays
+        // infinite and the scroll area is vertical-only, so text wraps to the pane
+        // width at any zoom (no horizontal scrolling).
+        let base = ui
+            .style()
+            .text_styles
+            .get(&egui::TextStyle::Monospace)
+            .map(|f| f.size)
+            .unwrap_or(12.0);
+        let size = (base * (1.0 + self.config.editor_zoom / 100.0)).max(4.0);
+
         egui::ScrollArea::vertical()
             .id_salt("editor")
             .auto_shrink([false, false])
@@ -723,6 +755,7 @@ impl App {
                 let resp = ui.add(
                     egui::TextEdit::multiline(&mut self.buffer)
                         .code_editor()
+                        .font(egui::FontId::monospace(size))
                         .desired_width(f32::INFINITY)
                         .desired_rows(30),
                 );

src/config.rs

@@ -20,6 +20,10 @@ pub struct Config {
     /// zeros to the width of the largest chapter number (e.g. "03." of 12).
     #[serde(default)]
     pub zero_pad_index: bool,
+    /// Editor text zoom as a percentage offset from the base size (0 = default,
+    /// +100 = double, -50 = half).
+    #[serde(default)]
+    pub editor_zoom: f32,
 }
 
 /// Default header/body separator, matching the manuscript drafting convention.
@@ -37,6 +41,7 @@ impl Default for Config {
             show_preview: false,
             draft_marker: default_marker(),
             zero_pad_index: false,
+            editor_zoom: 0.0,
         }
     }
 }