feat: implement contract scaffold templates with WASM hashes and spec export

[chidii]

Closes #187

Summary
Replaces TODO stubs in contractDevelopment.js with working WASM hash placeholders, spec JSON export, and downloadable scaffold bundles. Adds a "Scaffold" section to the Template Library UI with links to Stellar docs.

Changes

src/lib/contractDevelopment.js

• generateWasmHash(templateId) — deterministic hash placeholder for local dev
• exportContractSpec(templateId) — exports ABI-like JSON spec with methods and wasmHash
• generateContractReadme(templateId) — builds a starter README with build/deploy/test instructions
• downloadScaffold(templateId) — triggers browser download of spec + README + source bundle

src/components/templates/TemplateLibrary.jsx

• Added 'scaffold' category to filter bar
• New "Contract Scaffolds" section showing token/escrow/oracle templates with download buttons
• Links to Stellar smart contract documentation
• Shows "✓ Downloaded" feedback for 2 seconds after download

src/components/templates/TemplateDeployer.jsx

• Added "Quick Downloads" section with "Spec + README" download button and Stellar Docs link

Acceptance Criteria

• User can download starter spec + README for token/escrow/oracle patterns
• No runtime TODO throws
• Links to Stellar docs for deploying real contracts

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