fix: show funding warning when light-mode wallet has no xDAI

[crtahlin]

Summary

• Funding monitor now runs in both ultra-light and light modes. Previously it skipped entirely in light mode (if (currentMode === 'light') return), so when a node had swap-enable: true but an empty wallet, Bee crash-looped with no UI indication — the user saw "Starting Bee node…" forever.
• New needsFunding flag on /status endpoint. In light mode, the monitor polls wallet balance and sets this flag when xDAI is below threshold.
• UI shows funding banner when needsFunding is true, bypassing the startup overlay that was previously hiding it.

Closes crtahlin#1

Changed files

File	Change
src/funding-monitor.ts	Remove early return for light mode; add needsFunding state + getNeedsFunding() export; in light mode set flag based on wallet balance
src/status.ts	Add needsFunding: boolean to Status interface, sourced from getNeedsFunding()
ui/src/api/client.ts	Add needsFunding?: boolean to frontend Status type
ui/src/components/Layout.tsx	Include status?.needsFunding in funding warning condition; bypass startup overlay when needsFunding is true
CLAUDE.md	Update docs for funding-monitor and status modules
test/funding-monitor.spec.ts	New: tests for detectMode(), getNeedsFunding(), startMonitorIfNeeded()
test/status.spec.ts	New: tests for getStatus() with/without needsFunding

Test plan

• Backend unit tests pass (33 tests)
• UI unit tests pass (28 tests)
• TypeScript type check clean
• ESLint clean (0 errors)
• Manual test: start Nook with light-mode config + empty wallet → funding banner appears after ~15s
• Manual test: fund the wallet → banner disappears, Bee starts normally

AI-assisted implementation.