Per review: don't use is=, it reads as a hack (and it is one — is=-upgraded
built-ins can never host a shadow root, which is what caused the crash this PR
fixes in the first place). <hive-btn> is now a normal autonomous custom element
wrapping a real <button> inside its own shadow root, so it gets its shadow
encapsulation back (matching hive-dialog/hive-toast) instead of the document-
level stylesheet workaround from the previous commit.
delegatesFocus: true on the shadow root means .focus() on the host (what
modal.js calls for autofocus) reaches the inner button directly. The inner
button's native click is a composed event, so host-level click listeners
(what modal.js/themedPrompt already use) keep working unchanged.
modal.js: el('button', { is: 'hive-btn', ... }) -> el('hive-btn', { ... }) at
the one call site. dom.js: removed the is= special case from el() entirely —
it existed only to support this one now-gone usage. Build clean.
e5b307df1b