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>

Buffer.h

@@ -35,9 +35,12 @@
  */
 #pragma once
 
+#include <algorithm>
 #include <cstddef>
+#include <filesystem>
 #include <memory>
 #include <string>
+#include <unordered_set>
 #include <vector>
 #include <string_view>
 
@@ -48,6 +51,26 @@
 #include "Highlight.h"
 #include <mutex>
 
+// Edit mode determines which font class is used for a buffer.
+enum class EditMode { Code, Writing };
+
+// Detect edit mode from a filename's extension.
+inline EditMode
+DetectEditMode(const std::string &filename)
+{
+	std::string ext = std::filesystem::path(filename).extension().string();
+	std::transform(ext.begin(), ext.end(), ext.begin(), [](unsigned char c) {
+		return static_cast<char>(std::tolower(c));
+	});
+	static const std::unordered_set<std::string> writing_exts = {
+		".txt", ".md", ".markdown", ".rst", ".org",
+		".tex", ".adoc", ".asciidoc",
+	};
+	if (writing_exts.count(ext))
+		return EditMode::Writing;
+	return EditMode::Code;
+}
+
 // Forward declaration for swap journal integration
 namespace kte {
 class SwapRecorder;
@@ -484,6 +507,27 @@ public:
 	}
 
 
+	// Edit mode (code vs writing)
+	[[nodiscard]] EditMode GetEditMode() const
+	{
+		return edit_mode_;
+	}
+
+
+	void SetEditMode(EditMode m)
+	{
+		edit_mode_ = m;
+	}
+
+
+	void ToggleEditMode()
+	{
+		edit_mode_ = (edit_mode_ == EditMode::Code)
+			             ? EditMode::Writing
+			             : EditMode::Code;
+	}
+
+
 	void SetSyntaxEnabled(bool on)
 	{
 		syntax_enabled_ = on;
@@ -614,6 +658,9 @@ private:
 	std::unique_ptr<struct UndoTree> undo_tree_;
 	std::unique_ptr<UndoSystem> undo_sys_;
 
+	// Edit mode (code vs writing)
+	EditMode edit_mode_ = EditMode::Code;
+
 	// Syntax/highlighting state
 	std::uint64_t version_ = 0; // increment on edits
 	bool syntax_enabled_   = true;