fix(ui): render whole-page Handler dot as template data, not a tag

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Summary

Fixes #178. ui.Handler(jw, name, dot) documents that each request renders the named template with dot exposed through With.Dot, with no tag-eligibility restriction. In practice a primitive dot such as a string produced HTTP 200 with an empty body.

Whole-page rendering flowed through Template.render, which unconditionally tag-expands Dot. A string is rejected with tag.ErrIllegalTagType, so render returned before writing anything and ServeHTTP logged-and-discarded the error.

Changes

• Page dot is no longer a tag. pageTemplate gets its own JawsRender that looks up and executes the template directly (reusing Template.lookup/Template.execute), bypassing tag expansion. The page element cannot re-render itself — its JawsUpdate is a deliberate no-op — so deriving tag identity from the page dot was always inert. Partial ui.Template values are untouched and keep deriving tag identity from their dot.
• Render failures are observable. ServeHTTP wraps the ResponseWriter in a small recorder; a failure before any output (e.g. a missing template) now returns a clean 500 instead of a blank 200. Once bytes are committed the partial body is left as-is, matching the best-effort execution semantics documented on Template.

Acceptance criteria

• ui.Handler accepts ordinary html/template data, including strings.
• Whole-page rendering does not derive tag identity from the page dot.
• Render failures produce observable HTTP error behavior rather than a blank 200.
• Preserve partial ui.Template tag behavior.
• Add public-API regression tests under go test -race.

Testing

• go test -race ./lib/ui/ and go test -race ./... — pass
• go vet, gofmt -l, gofumpt -l, staticcheck ./lib/ui/ — clean
• go test -cover ./lib/ui/ — 100.0% of statements