Improve readability: brighter left-pane text and an editor contrast slider

The left file pane now boosts its widget text colours (toward white in dark
mode, black in light mode) so file names stay legible against the panel
background; the change is scoped to that panel only.

Add a "Contrast" slider beside Zoom that interpolates the editor text colour
from the theme default toward maximum contrast, persisted as
editor_text_contrast in the config. build_editor_job takes the computed colour
so underlined grammar/spelling runs match.

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

README.md

@@ -66,7 +66,10 @@ Terminal=false
 3. Create files with **＋ New**, edit on the right, `Ctrl+S` (or the Save button)
    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.
+   it), and the **Contrast** slider to brighten dim editor text (0% = theme
+   default; higher pushes the text toward white in dark mode / black in light
+   mode). 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