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>

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,
         }
     }
 }