refactor: one definition of the live connection list #963

"Every connection that can run a statement right now" (connected servers plus open SQLite files) is built in four places. They agree today, but where a run goes and whether a tab counts as orphaned (#944) both depend on them agreeing, so a change to one that misses the others would bring back bugs of that kind.

Reproduce

grep -n "activeConnections.map" src/hooks/useQueryExecution.ts src/hooks/useResultsFollowActiveTab.ts src/components/editor/SQLEditor.tsx

src/hooks/useQueryExecution.ts:45:    ...activeConnections.map((c) => c.id),
src/hooks/useResultsFollowActiveTab.ts:29:    ...activeConnections.map((c) => c.id),
src/components/editor/SQLEditor.tsx:32:    ...activeConnections.map((c) => c.id),

The fourth is liveConnectionIds() in src/lib/run-connection.ts, shared by the stores and useConnectionSwitching since #961.

Cause

The three hook and component copies are built from store selectors so they re-render when connections change; liveConnectionIds() reads store state once and can't replace them directly.

Fix

Add a useLiveConnectionIds hook next to liveConnectionIds() that subscribes to both stores and returns the same list, and use it in the three files. No behaviour change; the existing tests for those hooks and SQLEditor should pass unchanged.

While there: src/components/layout/useStartupReconnect.ts:20 uses a ! non-null assertion (t.profileId!), which src/CLAUDE.md rules out. Narrow with a type guard in the filter instead.