feat(explain): a click on a plan node shows every field the driver reported (#1164)

[ZhuchkaTriplesix]

Refs #1164

What was wrong

The plan tree showed each node's operation, relation, rows and cost bar. The other fields the driver reported (startup cost, filters, index names and so on) were parsed into PlanNode.details but not shown, which the issue asks for in the details panel.

Changes

• A click on a node selects it and shows a panel below the tree with its operation, relation, estimated and actual rows, cost, own cost and every reported field. A second click closes the panel.
• The selected node has a stronger outline; the most expensive node keeps its highlight.
• Widget test: a click shows the reported fields, the second click closes the panel.

Not in this PR

• EXPLAIN ANALYZE with buffers, which the issue asks for behind an explicit action.
• The acceptance check of a three-table join on a real database.

Not verified locally

Tests were not run locally, per the project rule.

[github-actions]

Grid scroll benchmark

metric	base	PR	change
p50	16.71 ms	16.91 ms	+1.2%
p90	21.42 ms	26.05 ms	+21.6% ⚠️
p99	54.25 ms	51.62 ms	-4.8%
stutters	295.00	297.00	+0.7%

Informational only (threshold 5%). Shared CI runners are noisy; re-run before trusting a single result.

[github-actions]

Grid scroll benchmark

metric	base	PR	change
p50	7.33 ms	7.23 ms	-1.4%
p90	15.69 ms	16.16 ms	+3.0%
p99	31.81 ms	35.95 ms	+13.0% ⚠️
stutters	336.00	333.00	-0.9%

Informational only (threshold 5%). Shared CI runners are noisy; re-run before trusting a single result.