uSync: structured diff for JSON/block changes

Repo: KevinJump/uSync

Replaced the word-diffed, pretty-printed JSON in the change-detail view with a structured diff — classifies each change (masked, single-sided, JSON, XML/text, word, scalar) and picks a rendering strategy per kind instead of running everything through diffWords
JSON/block diffs now match array entries by identity (key/udi/alias+culture) instead of index, so inserting one block into a block list reports one change instead of rewriting the whole array
Hand-rolled a line diff with collapsed unchanged runs for long text/XML, since Umbraco only re-exports diffWords
Replaced the change-detail table with expandable rows (collapsed by default once there's more than one change) and a raw-values toggle per JSON row
Fixed two accidental-precedence bugs and a case where Error/Warning notices were being diffed against an empty string
Opened KevinJump/uSync#1060