fix(settings): pick up changes made in another tab

[htcom-code]

Why

Every pc: key is shared by all tabs of the console, but each tab read them once at mount and never listened for the storage event. Lowering retention in one tab left another pruning the same IndexedDB to the old limit until it was reloaded, so the tab with the higher limit kept counting rows the other had already evicted.

Key	Read by	Cost of a stale value
pc:retention, pc:evictionFactor	useSettings	Storage: two tabs prune one IndexedDB to different limits
pc:pageSize, pc:sampleDelayMs	useSettings	Behaviour drifts between tabs
pc:metricsSort	usePersistentState	Display only
pc:theme	useTheme	Display only

What

• subscribeStored(keys, onChange) in use-persistent-state.ts listens for storage events on those keys. A null key means localStorage.clear(), and events from sessionStorage are ignored.
• useSettings reloads and re-validates all four settings on any of their keys. When nothing changed it keeps the old object, so effects keyed on settings do not restart. A save writes four keys and can arrive as four events; each one re-reads everything, so the tab ends up with the last value of each.
• usePersistentState picks up its key the same way, which covers pc:metricsSort. useTheme now builds on it for pc:theme, and the OS preference is still queried only when nothing is stored.

When a changed limit applies

From the next batch. The trace store reads its settings at eviction time, so an eviction already in progress finishes on the limit it started with. A new retention-limit scenario pins this: the limit drops from 8 to 6 between batches and the next arrival prunes to 6.

Verification

• 10 new tests (9 cross-tab, 1 retention). With the three hooks reverted, 5 of the 9 cross-tab tests fail. The other 4 are guards (validation, object identity, sessionStorage, unmount) and pass either way.
• Two browser tabs on the dev server: a write in one tab switched the other tab's theme, and its settings dialog showed retention 120000 and factor 4×, with no reload.
• npm run lint: 7 warnings, the same set as main
• npm test: 18 files, 212 tests pass