fix(web): address code review issues — ATB-59
- Fix GET /admin/themes/:rkey to call public /api/themes/:rkey instead
of nonexistent /api/admin/themes/:rkey; remove unused cookie variable
- Validate name before AppView PUT in save handler; redirect with error
if empty (prevents wasteful round-trip and unclear AppView message)
- Replace c.json() with redirect-on-error in reset-to-preset handler so
browser form POSTs show friendly error pages instead of raw JSON
- Add network failure test for GET /admin/themes/:rkey (500 unavailable)
- Add empty-name validation test for save handler
- Move ATB-59 plan docs to docs/plans/complete/