feat(workspace): Explain and Cancel in the SQL toolbar

[ZhuchkaTriplesix]

The two scenarios still open in #1051 tested features that had no UI: explainQuery and cancelQuery existed on the delegate, but nothing called them while a query was running.

• UI kit: new QueryaActionButton (lib/shared/widgets/querya_action_button.dart): labelled outline toolbar button with icon, loading spinner, isDestructive, tooltip. The SQL toolbar now uses it for History, Execute, Explain, Cancel, Diagram, Begin, Commit and Rollback instead of seven hand-built OutlineButtons. Documented in docs/ui-kit.md.
• Explain: shows the plan of the selection (or the whole editor) in the result grid, one line per row; does not run the query. Shown when delegate.supportsExplain (default true; off for generic extension drivers, which cannot explain).
• Cancel: appears next to Execute while a statement runs and calls delegate.cancelQuery(); the pending run then ends with the driver's cancellation error. supportsCancel is false for extension drivers, whose cancelQuery is a no-op.
• Tests: e2e_explain_and_cancel_query_test.dart (plan lines, empty editor, error, Cancel interrupts and the editor is usable again, buttons hidden for drivers without support), querya_action_button_test.dart. The fake delegate gets explainPlan, explainError, cancelAbortsGate and the two support flags.

Existing tests that tap Execute (F5) through OutlineButton still match, since the button wraps one.

Written without running locally; CI is the first run.

Closes #1051

[github-actions]

Grid scroll benchmark

metric	base	PR	change
p50	15.75 ms	16.78 ms	+6.5% ⚠️
p90	24.06 ms	26.31 ms	+9.4% ⚠️
p99	47.79 ms	56.45 ms	+18.1% ⚠️
stutters	310.00	290.00	-6.5%

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

[ZhuchkaTriplesix]

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