ui(trees): align Mongo sidebar hierarchy with other drivers

[ZhuchkaTriplesix]

Parent

Part of epic #490.

Problem

Mongo DB rows use left: 16, no Databases (N) group, iconSize: treeGroup (13). Redis/MySQL/PG use a Databases group at left: 20 + DB leaves at left: 16 with treeConnection (14).

Also leaf icon color drift: PG uses primary@0.5; MySQL/SQLite use mutedForeground — fix Mongo as part of alignment; note color unify may be a small sibling change.

Proposal

Align Mongo with Redis/MySQL: Databases group + treeConnection on DB rows + shared indent scale.

Files

• lib/features/connections/connections_panel_mongo.dart
• Compare: connections_panel_redis.dart, connections_panel_mysql.dart, connections_panel_pg_tree.dart

Acceptance

• Mongo Databases group + indent match sibling drivers
• DB row icon size uses treeConnection

[ZhuchkaTriplesix]

Shipped on dev via #511.