Refactor viewer JS into domain subdirectories (#932)

Group the flat module files under src/viewer/assets/lib/ into
selection/, sections/, events/, ui/, and features/ subdirectories.
Core orchestration files (app.js, data.js, viewer_core.js,
viewer_api.js) and the script.js entry point stay at the lib root
since they are referenced by index.html and have standalone
site-viewer counterparts.

All relative imports were rewritten by path resolution, the
site/viewer/lib symlink tree regenerated to mirror the new layout,
and test import paths plus the site standalone script.js entry
updated accordingly.

Co-authored-by: Claude <noreply@anthropic.com>

Author: thinkingfish

site/viewer/lib/cgroup_selector.js

@@ -0,0 +1 @@
+../../../../src/viewer/assets/lib/events/event_form.js

site/viewer/lib/chart_controls.js

@@ -0,0 +1 @@
+../../../../src/viewer/assets/lib/events/events_store.js

site/viewer/lib/controls.js

@@ -0,0 +1 @@
+../../../../src/viewer/assets/lib/features/cgroup_selector.js

site/viewer/lib/event_form.js

@@ -0,0 +1 @@
+../../../../src/viewer/assets/lib/features/explorers.js