fix: use subsection start_time and add tail section in HTML converter

Unified HTML subsection timing with PowerPoint approach:
- Use subsection.start_time instead of always starting from 0.0
- Add tail section for content after last subsection
- Matches Qt presenter and PowerPoint behavior

This fixes timing issues in HTML where subsections were always
showing accumulated content from the beginning instead of just
the subsection's time range.

Author: krystophny

manim_slides/convert.py

@@ -587,6 +587,7 @@ def _iter_slide_sections(self, slide_config: SlideConfig) -> list[dict[str, Any]
             ]
 
         sections = []
+        last_end = 0.0
         for subsection in slide_config.subsections:
             sections.append(
                 {
@@ -596,10 +597,24 @@ def _iter_slide_sections(self, slide_config: SlideConfig) -> list[dict[str, Any]
                     "notes": f"{slide_config.notes}\n\n{subsection.name}"
                     if slide_config.notes and subsection.name
                     else subsection.name or slide_config.notes,
-                    "start_time": 0.0,  # Always start from beginning to show accumulated content
+                    "start_time": subsection.start_time,
                     "end_time": subsection.end_time,
                 }
             )
+            last_end = subsection.end_time
+
+        video_duration = get_duration_seconds(slide_config.file)
+        if video_duration - last_end > 1e-3:
+            sections.append(
+                {
+                    "file": slide_config.file,
+                    "loop": slide_config.loop,
+                    "auto_next": False,
+                    "notes": slide_config.notes,
+                    "start_time": last_end,
+                    "end_time": video_duration,
+                }
+            )
         return sections
 
     def convert_to(self, dest: Path) -> None:  # noqa: C901