feat(examples): structured error handling and user messaging

[danielrx-echo]

Summary

• Adds messages.ts to all 4 example apps as a single source of truth for all user-facing strings (entity state, purchase readiness, errors, contract errors)
• Adds parseError.ts to each app with chain-specific error parsers: parseEVMError (viem cause-chain unwrapping, ABI error name lookup), parseSVMError (Anchor error code extraction via regex from logs, IDL-derived friendly message map), and parsePermitError (API/network errors)
• Adds ErrorToast component with auto-dismiss and manual close
• Splits purchase() try-catch in CommitCard so permit API errors use parsePermitError and transaction errors use the chain-specific parser
• Uses granular loading state ("idle" | "permit" | "submitting") so the commit button shows "Generating permit..." vs "Submitting..." vs "Loading..."
• Disables the commit button when isWrongChain (EVM) or notInCommitmentStage, with updated messaging

Test plan

• Connect wallet on wrong network (EVM) — commit button should be disabled with "Wrong network — switch to Base Sepolia to commit." banner
• Click Commit on correct network — button should show "Generating permit..." then "Submitting..."
• Reject wallet prompt — toast should show "Transaction cancelled."
• Simulate network error during permit generation — toast should show network error message
• Simulate contract revert — toast should show the mapped ABI/program error message