fix(frontend): ensure isLive is false whenever polling is the active source (Closes #904)

[waterWang]

Summary

Fixes #904 — the useBalanceStream hook could leave isLive: true on the UI even when polling was the active data source, or carry a stale "live" badge across account switches.

Root cause

startPolling() was never asserting setIsLive(false). It is reached from several paths that skip closeSource():

• No EventSource support
• No SEP-10 token after refresh
• Ticket-fetch failure
• new EventSource(...) construction failure

In all of those the consumer's isLive indicator was left at whatever the previous session/account had set it to, so a "live" badge could persist while polling was actually running.

Changes

frontend/lib/useBalanceStream.ts

• startPolling now explicitly calls setIsLive(false) so polling as the active source is always reflected in the consumer-facing indicator.

frontend/__tests__/balance-stream.test.tsx

• Added 3 targeted tests for #127 — SSE Endpoint Fallback to Polling Never Clears on Reconnect Failure #904:
  1. isLive drops to false after a stream error hands over to polling.
  2. A stale reconnect (late balance event) from an old account cannot flip isLive back on.
  3. Account switches cleanly reset the live flag before the new stream delivers anything.
• Fixed pre-existing stale assertions (token= → ticket= after the short-lived-ticket auth change) and added an apiFetch mock so SSE tests can actually construct EventSource instances.

Verification

• npm test -- __tests__/balance-stream.test.tsx → 16/16 passing
• lint + type-check run clean

[github-actions]

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