feat(frontend): add polling fallback and reconnect logic for SSE (#577)

[onyillto]

• Fall back to setInterval polling every 30s when SSE connection fails
• Add exponential backoff auto-reconnect after connection drops
• Expose polling status for "Live" / "Refreshing every 30s" UI indicator
• Guard against SSR by checking window.EventSource availability

closes:577

[ogazboiz]

The codebase issues on main have been resolved and all CI checks are passing now. Please rebase your branch to pull in the latest changes before continuing. Thanks for your patience.

[ogazboiz]

Critical syntax error in WalletProvider.tsx: disconnectWallet function is defined inside signTransaction, replacing its closing throw statement and brace. This won't compile. Branch name pooling is a typo for polling. The useSSE polling changes are fine but the WalletProvider is broken.

[ogazboiz]

heads up, a few important changes just landed on main that affect your PR:

1. axios pinned to 1.13.5 - there's an active supply chain attack on axios 1.14.1 (pulls in confirmed malware). we added overrides in all package.json files to block it.

2. CI now runs a supply chain audit before backend/frontend jobs. if your lockfile has a compromised package, CI will fail with a clear error.

3. backend test fixes - loanEndpoints tests now use valid Stellar addresses and base64 strings. if your PR was failing backend CI but you didn't touch backend code, this should fix it after rebase.

please rebase on latest main:

git fetch upstream
git rebase upstream/main
git push --force-with-lease