fix: checksum address validation, polling-abort safety (#143, #142, #140, #138)

[christabel888]

Summary

• security: Add Stellar ed25519 address checksum validation on frontend #143 — Added validateStellarAddress() in lib/stellar.ts, which classifies an invalid address as either format (wrong shape entirely) or checksum (right length/prefix/alphabet but a bad CRC-16 — the classic single-mistyped-character near-miss). Wired it into the create-stream form and the batch-upload CSV row validation so each surfaces a specific "Invalid checksum" message instead of a generic one. StrKey.isValidEd25519PublicKey() was already the underlying boolean check on both surfaces; this adds the distinct messaging the acceptance criteria calls for. Added unit + component tests covering valid/format/checksum cases.
• bug: Potential memory leak from polling intervals on component unmount #142 — useStreams/useStream (hooks/use-streams.ts) already created exactly one polling interval per mount and cleared it correctly on unmount/StrictMode remount, with per-request AbortController already cancelling stale in-flight requests before starting a new one. The one gap against the acceptance criteria: nothing aborted the still in-flight request specifically on unmount. Added that. Added tests pinning down the interval lifecycle (one tick per pollInterval, cleared exactly once on unmount, no orphaned fetches firing after unmount).
• bug: top_up() doesn't update linear_amount field, causing stale stream metadata #140 — Verified top_up() in contracts/streaming/src/lib.rs already updates linear_amount on both branches (past-cliff re-anchor and pre-cliff growth), and this is already covered by test_top_up_before_start_time_updates_deposit_and_rate (assert_eq!(stream.linear_amount, total + additional)). No code change made — already resolved on main.
• bug: Duplicate useCallback definitions in use-contract.ts overwrite each other #138 — Verified hooks/use-contract.ts has no duplicate useCallback definitions on main; createStream, withdraw, and estimateFee are each defined exactly once. No code change made — already resolved on main.

Test plan

• vitest run __tests__/lib/stellar-address-validation.test.ts __tests__/components/create-form.test.tsx __tests__/components/batch-create-page.test.tsx __tests__/hooks/use-streams.test.ts
• Manually mistype one character of a valid recipient address on the create-stream form and on a batch-upload CSV row; confirm the checksum-specific message appears on both.

Closes #143
Closes #142
Closes #140
Closes #138

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