Add proportional fonts, edit modes, and TOML config

- Add three proportional serif fonts: Crimson Pro, ET Book, Spectral
- Fix text rendering for variable-width fonts: selection, cursor,
  mouse click mapping, search highlights, and syntax-colored text
  now use pixel-accurate measurement via ImGui::CalcTextSize()
- Add per-buffer edit mode (code/writing) with auto-detection from
  file extension (.txt, .md, .rst, .org, .tex default to writing)
- Add C-k m keybinding and :mode command to toggle edit modes
- Switch config format from INI to TOML (kge.toml), with legacy
  INI fallback; vendor toml++ v3.4.0
- New config keys: font.code and font.writing for per-mode defaults
- Add font tab completion for ImGui builds
- Add tab completion for :mode command
- Update help text, themes.md, and add CONFIG.md
- Bump version to 1.10.0

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

fonts/FontRegistry.cc

@@ -45,6 +45,16 @@ InstallDefaultFonts()
 		BrassMonoCode::DefaultFontBoldCompressedData,
 		BrassMonoCode::DefaultFontBoldCompressedSize
 	));
+	FontRegistry::Instance().Register(std::make_unique<Font>(
+		"crimsonpro",
+		CrimsonPro::DefaultFontRegularCompressedData,
+		CrimsonPro::DefaultFontRegularCompressedSize
+	));
+	FontRegistry::Instance().Register(std::make_unique<Font>(
+		"etbook",
+		ETBook::DefaultFontRegularCompressedData,
+		ETBook::DefaultFontRegularCompressedSize
+	));
 	FontRegistry::Instance().Register(std::make_unique<Font>(
 		"fira",
 		FiraCode::DefaultFontRegularCompressedData,
@@ -95,6 +105,11 @@ InstallDefaultFonts()
 		SpaceMono::DefaultFontRegularCompressedData,
 		SpaceMono::DefaultFontRegularCompressedSize
 	));
+	FontRegistry::Instance().Register(std::make_unique<Font>(
+		"spectral",
+		Spectral::DefaultFontRegularCompressedData,
+		Spectral::DefaultFontRegularCompressedSize
+	));
 	FontRegistry::Instance().Register(std::make_unique<Font>(
 		"syne",
 		Syne::DefaultFontRegularCompressedData,