fix: validate daily focus input

[saurabhhhcodes]

Summary

• sanitize and cap daily focus goal text before persistence
• validate requested dates across POST, GET, and DELETE
• add focused route tests for invalid input and sanitized saves

Closes #3164

Validation

• npx vitest run test/daily-focus-validation.test.ts --environment node
• npm run type-check
• npx eslint src/app/api/daily-focus/route.ts test/daily-focus-validation.test.ts
• git diff --check

Note: the default jsdom Vitest harness is incompatible with the repository's installed undici/jsdom combination under Node 26; the server-route suite passes in the Node environment.

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

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• level:beginner — 20 pts
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• level:advanced — 55 pts
• level:critical — 80 pts

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• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

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Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[github-actions]

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