feat(explain): visual query plan (Plan | Text) for PostgreSQL, MySQL, SQLite #1164

Problem

Explain shows the plan as lines of text in the result grid (QUERY PLAN column, generic_sql_workspace.dart:590). Finding the expensive node in a nested plan means reading indentation.

Scope

• Plan formats: PostgreSQL EXPLAIN (FORMAT JSON) (and ANALYZE, BUFFERS behind an explicit Explain analyze action with the destructive-query rules, since it executes the query), MySQL EXPLAIN FORMAT=JSON / EXPLAIN ANALYZE (8.0.18+), SQLite EXPLAIN QUERY PLAN (id, parent, detail already form a tree).
• Plan view: a tree (top-down) of nodes with operation, relation, rows (estimated / actual), cost / time; node colour by share of total cost; the most expensive path highlighted. Click a node: details panel with all fields.
• Text view kept (today's lines), switch Plan | Text.

Acceptance

• PostgreSQL, MySQL, SQLite: a join of 3 tables renders as a tree with the right parent / child structure.
• The most expensive node is highlighted.
• Parser unit tests on recorded plans per dialect.