Batch fix: wallet error boundary, resize debounce, dead error variant, mocked test auth

[Lost-Z]

⚠️ Warning

This PR was implemented but not tested or compiled/run — per explicit request, no cargo test, npm test, or manual verification was performed. Please build/test before merging, especially the Rust contract test changes in test.rs (event-topic decoding via Symbol::try_from_val), which use a soroban-sdk testutils pattern that has not been compiled in this session.

Summary

Bundles four independent fixes into one PR:

• Add Error Boundary Wrapper Around WalletConnect for Freighter Extension Failures #301 — Add WalletErrorBoundary and wrap WalletConnect in the navbar with it, so a Freighter extension throw during detection/connection shows an inline fallback with a retry button instead of crashing the whole app. Errors are logged via console.error in componentDidCatch.
• Fix NetworkBackground Resize Handler Causing Layout Thrashing on Window Resize #300 — NetworkBackground's resize handler now debounces to 200ms (was a 100ms throttle) and skips the resize/recalibration work entirely when the physical canvas size hasn't actually changed. Particle positions continue to be preserved (scaled) across resizes via the existing recalibrateParticles.
• Eliminate Dead-Code NoFundsToRefund Error Variant From ABI Surface #302 — Removed the dead-code NoFundsToRefund error variant from TrellisError (discriminant 6, never returned by any codepath). Discriminant 6 is left vacant with a comment explaining why, per the enum's append-only stability rule. No CLI or frontend code referenced this variant.
• Fix No-Op Event Assertion in Integration Tests -- Mocked Auth Silently Skips Verification #303 — Replaced all env.mock_all_auths() calls in contracts/trellis_core/src/test.rs with targeted auth_as(&env, &payer) calls (verified init only requires the payer's auth), and removed two mock-auth calls guarding extend_agreement_ttl, which has no require_auth() gate at all. Replaced the always-true all_events.is_empty() || all_events.len() >= 4 assertion in test_happy_path with a real check that the escrow contract's own events fire in order (created → locked → submitted → released), decoding each event's topic-0 symbol.

Test plan

• cargo test -p trellis_core (contract changes are unverified — see warning above)
• npm run build / npm test in frontend/
• Manually simulate a Freighter throw and confirm the wallet fallback + retry works
• Rapidly resize the window and confirm no layout thrashing, particles preserved

Closes #301
Closes #300
Closes #302
Closes #303

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