Remember the last-used side panel tab #2749

The row side panel resets to Overview every time it's opened. If you're working through a list of rows and want Column Values, you have to re-click that tab on every single row.

Instead, remember whichever tab the user last selected and open there next time. If that tab doesn't exist for the row being opened (e.g. last used Column Values, now opening a trace row), fall back to today's default logic.

Notes:

• Tab state currently lives in useSidePanelStack and starts as null on each open. Persist the last selection to localStorage and seed from it.
• reconcileTab (packages/app/src/hooks/useSidePanelStack.ts:36) already falls back to defaultTab when the requested tab isn't in availableTabs, so no new fallback logic is needed.
• Explicit navigations that target a specific tab (e.g. clicking into a trace) should still win over the remembered value.