fix(chat): support Shift+Enter for multi-line input and proper newline display

[nimitbhardwaj]

Summary

Fixes #1141 - Add Shift+Enter to insert new line in chat input

Changes

• Web UI textarea: Modified @keydown.enter handler to only call preventDefault() when Shift is NOT held, allowing Shift+Enter to insert newlines naturally
• escapeHtml function: Added .replace(/\n/g, '
  ') to convert newlines to
  tags so user messages display with proper line breaks in the chat UI

Testing

• cargo clippy --workspace --all-targets -- -D warnings passes
• cargo test --workspace passes
• Live integration tested (if applicable)

Security

• No new unsafe code
• No secrets or API keys in diff
• User input validated at boundaries (sanitization already exists in sanitize_user_input() / sanitize_text())