Add leuchtturm theme, font zoom, syntax palette fixes

Themes:
- Add leuchtturm theme (fountain pen ink on cream paper, brass/leather dark)
- Add per-theme syntax palettes for leuchtturm, tufte, and everforest
- Fix static inline globals giving each TU its own copy of gCurrentTheme
  and gBackgroundMode (changed to inline for proper C++17 linkage)
- :background with no args now shows current mode

Font zoom:
- CMD-=/CMD--/CMD-0 to increase/decrease/reset font size

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Command.h

@@ -113,6 +113,10 @@ enum class CommandId {
 	CenterOnCursor, // center the viewport on the current cursor line (C-k k)
 	// GUI: open a new editor window sharing the same buffer list
 	NewWindow,
+	// GUI: font size controls
+	FontZoomIn,
+	FontZoomOut,
+	FontZoomReset,
 };