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>

ImGuiFrontend.cc

@@ -76,7 +76,9 @@ static void
 update_editor_dimensions(Editor &ed, float disp_w, float disp_h)
 {
 	float row_h = ImGui::GetTextLineHeightWithSpacing();
-	float ch_w  = ImGui::CalcTextSize("M").x;
+	// Use average character width rather than "M" (the widest character)
+	// so that column count is reasonable for proportional fonts too.
+	float ch_w  = ImGui::CalcTextSize("abcdefghijklmnopqrstuvwxyz").x / 26.0f;
 	if (row_h <= 0.0f)
 		row_h = 16.0f;
 	if (ch_w <= 0.0f)