feat(editor): add theme and auto_sync preset prefs with rotate-on-Enter

Generalise the palette so Enter advances any pref to its next value and
wraps; a boolean toggle is the two-option case. Both string prefs share a
next_option(current, &OPTIONS) helper (off-list values snap to the head).

- theme (light/dark): rendered as a single whole-frame Frame::invert at the
  end of draw(), so text, selection, caret, panel and palette flip together.
- auto_sync: cycles 2m/5m/10m/15m/30m. Set-ahead only — still read by
  nothing until the v0.7 periodic push.

toggle_pref -> cycle_pref; palette hint "Enter toggle" -> "Enter change".

display/src/lib.rs

@@ -76,6 +76,16 @@ impl Frame {
     pub fn bytes(&self) -> &[u8] {
         &self.buf
     }
+
+    /// Flip every pixel black↔white across the whole framebuffer. The editor
+    /// draws its native black-ink-on-white-paper frame, then calls this once at
+    /// the end for the dark theme — so text, selection, caret, panel and palette
+    /// all invert together and each stays legible against the flipped ground.
+    pub fn invert(&mut self) {
+        for b in &mut self.buf {
+            *b = !*b;
+        }
+    }
 }
 
 impl OriginDimensions for Frame {