style/standardize smart contract spelling

[Earnwithalee7890]

What changed? Why?
Standardized "smart-contract" (hyphenated) to "smart contract" (two words) in documentation text.
While hyphenating compound adjectives is grammatically acceptable, the Base documentation consistently uses "smart contract" as two separate words, even in adjectival positions (e.g., "smart contract architecture"). This change ensures consistency across all docs.

Notes to reviewers

• Updated instances in terms-of-service.mdx ("smart contract failures") and base-services-hub.mdx ("smart contract architecture", "smart contract audits").
• Only modified user-facing text descriptions, not code variables or technical identifiers.
• Aligns with the established style pattern throughout Base documentation.

How has it been tested?

• grep search to identify all hyphenated "smart-contract" instances.
• Cross-referenced with existing usage patterns in the documentation to confirm two-word form is standard.
• Verified changes render correctly in Mintlify preview.

[cb-heimdall]

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