Before background blur got its own shape attribute, the `:blur` attribute
accepted both `:layer-blur` and `:background-blur` types, so the editor and
the plugin API could save a background blur under `:blur`. The shape schema
was later tightened to only allow `:layer-blur` on `:blur`, but no migration
moved the existing values, so those files fail server schema validation.
Add migration 0029: when a shape has a `:blur` map with `:type
:background-blur`, move it to the `:background-blur` attribute. When the
shape already has a `:background-blur`, keep it and drop the mis-typed
`:blur`. The migration walks both pages and components.
Closes#11904
AI-assisted-by: deepseek-v4.1-flash
dfd28b1e57