🛡️ Sentinel: Add input length limits to prevent DoS

[google-labs-jules]

Added input length limits to WelcomeScreen.tsx to prevent DoS attacks via large payloads.

• Added MAX_INPUT_LENGTHS to constants.ts.
• Enforced maxLength on name, systemInstruction, and greeting inputs.
• Added visual character counters to input fields.
• Updated components/WelcomeScreen.test.tsx to verify input truncation.
• Added constants.test.ts to verify security limits.
• Ensured index.html loads index.tsx correctly.

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PR created automatically by Jules for task 16746004784153298903 started by @tblakex01

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