perf(mongodb): do not block document list on countDocuments #876

Summary

MongoDocumentsView._load awaits a full count command before a bounded find (limit 25). Exact count scales with collection size / filter; first paint waits on both RTTs. Same class as SQL Table Browser COUNT(*) that #788 / #799 / #807 replaced with estimates or skip-count.

Scope

• Paint the first page from find without waiting on exact count.
• Use $collStats / estimated count, or “page full ⇒ maybe more” like SQLite browse.
• Filtered counts can stay exact in the background after first paint.

Out of scope

• Changing page size 25.
• Unbounded find API if limit omitted (footgun; not a current UI caller).