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