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>

src/config.rs

@@ -24,6 +24,11 @@ pub struct Config {
     /// +100 = double, -50 = half).
     #[serde(default)]
     pub editor_zoom: f32,
+    /// Editor text contrast as a percentage: 0 = the theme's default text color,
+    /// 100 = maximum contrast against the background (white in dark mode, black
+    /// in light mode). Higher values make dim editor text easier to read.
+    #[serde(default)]
+    pub editor_text_contrast: f32,
     /// URL scheme for the LanguageTool server: `http` (local) or `https`.
     #[serde(default = "default_languagetool_scheme")]
     pub languagetool_scheme: String,
@@ -78,6 +83,7 @@ impl Default for Config {
             draft_marker: default_marker(),
             zero_pad_index: false,
             editor_zoom: 0.0,
+            editor_text_contrast: 0.0,
             languagetool_scheme: default_languagetool_scheme(),
             languagetool_host: default_languagetool_host(),
             languagetool_port: default_languagetool_port(),