perf(mysql): offload table browse row→string conversion from UI isolate

[ZhuchkaTriplesix]

Parent

Part of #414

Problem

MysqlTableView converts every cell with sync List.generate on the UI isolate. SQL workspace already uses compute / streaming — table browse does not.

Evidence

• lib/features/mysql/mysql_table_view.dart (~147–157, ~188–189, ~228)

Acceptance

• Table browse reuses the same off-main conversion path as SQL workspace
• No UI jank on wide × tall browse loads within max-rows cap
• Shared helper preferred over duplicated convert logic

Suggested fix

Reuse convertMysqlResultRowsToStrings via compute (or stream convert).

[ZhuchkaTriplesix]

Shipped on dev via #439 (Closes did not auto-close because PRs targeted dev).