Add QtFrontend plans.

docs/plans/qt-frontend.md

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+Based on the project structure and the presence of files like
+`imgui.ini`, `GUIFrontend.h`, and `TerminalFrontend.h`, here is an
+analysis of the difficulty and challenges involved in adding a GTK or Qt
+version of the GUI.
+
+### **Executive Summary: Difficulty Level - Moderate**
+
+The project is well-architected for this task. It already supports
+multiple frontends (Terminal vs. GUI), meaning the "Core Logic" (
+Buffers, Syntax, Commands) is successfully decoupled from the "View" (
+Rendering/Input). However, the specific move from an **Immediate Mode**
+GUI (likely Dear ImGui, implied by `imgui.ini` and standard naming
+patterns) to a **Retained Mode** GUI (Qt/GTK) introduces specific
+architectural frictions regarding the event loop and state management.
+
+---
+
+### **1. Architectural Analysis**
+
+The existence of abstract interfaces—likely `Frontend`, `Renderer`, and
+`InputHandler`—is the biggest asset here.
+
+* **Current State:**
+    * **Abstract Layer:** `Frontend.h`, `Renderer.h`, `InputHandler.h`
+      likely define the contract.
+    * **Implementations:**
+        * `Terminal*` files implement the TUI (likely ncurses or VT100).
+        * `GUI*` files (currently ImGui) implement the graphical
+          version.
+* **The Path Forward:**
+    * You would create `QtFrontend`, `QtRenderer`, `QtInputHandler` (or
+      GTK equivalents).
+    * Because the core logic (`Editor.cc`, `Buffer.cc`) calls these
+      interfaces, you theoretically don't need to touch the core text
+      manipulation code.
+
+### **2. Key Challenges**
+
+#### **A. The Event Loop Inversion (Main Challenge)**
+
+* **Current (ImGui):** Typically, the application owns the loop:
+  `while (running) { HandleInput(); Update(); Render(); }`. The
+  application explicitly tells the GUI to draw every frame.
+* **Target (Qt/GTK):** The framework owns the loop: `app.exec()` or
+  `gtk_main()`. The framework calls *you* when events happen.
+* **Difficulty:** You will need to refactor `main.cc` or the entry point
+  to hand over control to the Qt/GTK application object. The Editor's "
+  tick" function might need to be connected to a timer or an idle event
+  in the new framework to ensure logic updates happen.
+
+#### **B. Rendering Paradigm: Canvas vs. Widgets**
+
+* **The "Easy" Way (Custom Canvas):**
+    * Implement the `QtRenderer` by subclassing `QWidget` and overriding
+      `paintEvent`.
+    * Use `QPainter` (or Cairo in GTK) to draw text, cursors, and
+      selections exactly where the `Renderer` interface says to.
+    * **Pros:** Keeps the code similar to the current ImGui/Terminal
+      renderers.
+    * **Cons:** You lose native accessibility and some native "feel" (
+      scrolling physics, native text context menus).
+* **The "Hard" Way (Native Widgets):**
+    * Trying to map an internal `Buffer` directly to a `QTextEdit` or
+      `GtkTextView`.
+    * **Difficulty:** This is usually very hard because the Editor core
+      likely manages its own cursor, selection, and syntax highlighting.
+      Syncing that internal state with a complex native widget often
+      leads to conflicts.
+    * **Recommendation:** Stick to the "Custom Canvas" approach (drawing
+      text manually on a surface) to preserve the custom editor
+      behavior (vim-like modes, specific syntax highlighting).
+
+#### **C. Input Handling**
+
+* **Challenge:** Mapping Qt/GTK key events to the internal `Keymap`.
+* **Detail:** ImGui and Terminal libraries often provide raw scancodes
+  or simple chars. Qt/GTK provide complex Event objects. You will need a
+  translation layer in `QtInputHandler::keyPressEvent` that converts
+  `Qt::Key_Escape` -> `KKey::Escape` (or your internal equivalent).
+
+### **3. Portability of Assets**
+
+#### **Themes (Colors)**
+
+* **Feasibility:** High.
+* **Approach:** `GUITheme.h` likely contains structs with RGB/Hex
+  values. Qt supports stylesheets (QSS) and GTK uses CSS. You can write
+  a converter that reads your current theme configuration and generates
+  a CSS string to apply to your window, or simply use the RGB values
+  directly in your custom `QPainter`/Cairo drawing logic.
+
+#### **Fonts**
+
+* **Feasibility:** Moderate.
+* **Approach:**
+    * **ImGui:** Usually loads a TTF into a texture atlas.
+    * **Qt/GTK:** Uses the system font engine (Freetype/Pango).
+    * **Challenge:** You won't use the texture atlas anymore. You will
+      simply request a font family and size (e.g.,
+      `QFont("JetBrains Mono", 12)`). You may need to ensure your custom
+      renderer calculates character width/height metrics correctly using
+      `QFontMetrics` (Qt) or `PangoLayout` (GTK) to align the grid
+      correctly.
+
+### **4. Summary Recommendation**
+
+If you proceed, **Qt** is generally considered easier to integrate with
+C++ projects than GTK (which is C-based, though `gtkmm` exists).
+
+1. **Create a `QtFrontend`** class inheriting from `Frontend`.
+2. **Create a `QtWindow`** class inheriting from `QWidget`.
+3. **Implement `QtRenderer`** that holds a pointer to the `QtWindow`.
+   When the core calls `DrawText()`, `QtRenderer` should queue that
+   command or draw directly to the widget's paint buffer.
+4. **Refactor `main.cc`** to instantiate `QApplication` instead of the
+   current manual loop.