Fix proportional font rendering with pixel-based horizontal scroll

The GUI renderer had two competing horizontal scroll systems: a
character-based one (coloffs × space_w) and a pixel-based one. For
proportional fonts the character-based system used "M" width to
calculate viewport columns, triggering premature scrolling at ~50%
of the actual display width.

Switch the GUI renderer to purely pixel-based horizontal scrolling:
- Remove coloffs↔ImGui scroll_x bidirectional sync
- Measure rx_to_px from column 0 (absolute) instead of from coloffs
- Draw full expanded lines; let ImGui clip via its scroll viewport
- Report content width via SetCursorPosX+Dummy for the scrollbar
- Use average character width for cols estimate (not "M" width)

The terminal renderer continues using coloffs correctly—no changes
needed there.

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

CMakeLists.txt

@@ -4,7 +4,7 @@ project(kte)
 include(GNUInstallDirs)
 
 set(CMAKE_CXX_STANDARD 20)
-set(KTE_VERSION "1.10.1")
+set(KTE_VERSION "1.11.0")
 
 # Default to terminal-only build to avoid SDL/OpenGL dependency by default.
 # Enable with -DBUILD_GUI=ON when SDL2/OpenGL/Freetype are available.