Add DateTime min/max cross-validation and generalize minExceedsMax

[JeroenDeDauw]

Fixes #730

DateTimeAttributesEditor.vue now refuses to emit an update when the
minimum timestamp exceeds the maximum and surfaces the existing
neowiki-property-editor-min-exceeds-max message on the offending
field, matching the behavior the number editor already had.

The comparison is a local 6-line helper that parses each input via
Date.parse and treats empty or unparseable input as "no value" so
typos don't produce spurious range errors. I considered generalizing
the existing minExceedsMax helper to take a parser callback (the
original plan in #730), but that turned out to be net-negative: the
two existing consumers (NumberAttributesEditor, TextAttributesEditor)
both already reduce to Number(...), so the generalization
parameterized a variation that didn't actually vary between current
consumers, while adding indirection at every call site. Keeping the
shared helper untouched and inlining DateTime's check reads more
simply.

Result of extensive back-and-forth with @JeroenDeDauw (including an explicit
course-correction away from the minExceedsMax refactor originally proposed
in #730).
Context: the NeoWiki codebase and the review of #685.
_{Written by Claude Code, Opus 4.7}