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.cc

@@ -1357,6 +1357,10 @@ cmd_background_set(const CommandContext &ctx)
 	std::transform(mode.begin(), mode.end(), mode.begin(), [](unsigned char c) {
 		return (char) std::tolower(c);
 	});
+	if (mode.empty()) {
+		ctx.editor.SetStatus(std::string("Background: ") + kte::BackgroundModeName());
+		return true;
+	}
 	if (mode != "light" && mode != "dark") {
 		ctx.editor.SetStatus("background: expected 'light' or 'dark'");
 		return true;