fix(#1138): fix SSE EventSource leak and polling race condition (duplicate, conflicts resolved)

[ionfwsrijan]

Duplicate of #1174 with merge conflicts resolved and rebased onto latest main.

Problem

1. SSE EventSource leak: On reconnect, the es.onerror handler could fire
   for the old EventSource after a new connection was established, triggering
   spurious reconnection loops and duplicate EventSources.
2. Polling race: setInterval(loadTasks, 5000) in Scans.tsx doesn't await
   the async loadTasks(). If a request takes longer than 5s, overlapping
   requests pile up, potentially corrupting state.

Solution

• Added a versionRef (generation counter) to useTaskSubscription.ts that
  increments on every connectSSE() call and every cleanupAll()
• All SSE callbacks (status, phase, output, onerror, onopen) now check
  versionRef.current === expectedVersion before acting — stale callbacks from
  previous connections are silently ignored
• In Scans.tsx, replaced setInterval with chained setTimeout:
  scheduleNextPoll() awaits loadTasks() before scheduling the next tick
• Renamed intervalRef → pollingTimerRef for clarity

Files changed

frontend/src/hooks/useTaskSubscription.ts, frontend/src/pages/Scans.tsx

Closes #1138

[ionfwsrijan]

@utksh1 Please review this now

[ionfwsrijan]

@utksh1 Please add other labels