Add permission checks to prototype flow and flex layout operations
that were missing them, allowing plugins to modify flows and layout
structure without explicit user permission.
Changes:
- page.cljs: Add content:write checks to flow-proxy (name,
startingBoard setters, remove) and page-proxy (createFlow,
removeFlow)
- flex.cljs: Add content:write checks to flex-layout-proxy
(remove, appendChild)
Follows the established pattern from tokens.cljs, shape.cljs,
and library.cljs.
Relates to #11137
AI-assisted-by: qwen3.7-plus
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