Refactor web build process and update documentation

- Removed Node.js build artifacts from .gitignore and adjusted Makefile to reflect changes in web UI build process, now using server-rendered Go templates instead of React.
- Updated README to clarify the new web UI architecture and output formats, emphasizing the removal of the Node.js build step.
- Added a command to set the number of frames per render task in manager configuration, enhancing user control over rendering settings.
- Improved Gitea workflow by removing unnecessary npm install step, streamlining the CI process.

internal/database/migrations/000002_add_task_frame_end.up.sql

@@ -0,0 +1,2 @@
+-- Add frame_end to tasks for range-based render tasks (NULL = single frame, same as frame)
+ALTER TABLE tasks ADD COLUMN frame_end INTEGER;