Add Xquik CSV sentiment imports

[kriptoburak]

Summary:

• Add reviewed Xquik CSV export support to the existing batch sentiment upload flow.
• Detect common text columns such as text, tweet, Tweet, tweet_text, full_text, content, and body.
• Let users download scored CSV results after TextBlob or VADER analysis.

Target improvements:

• Handle empty CSV files, parse errors, missing text columns, and blank text rows with clear Streamlit errors.
• Reuse a single VADER analyzer instance and warn on blank single-tweet input.
• Replace placeholder clone instructions with the real repository path and runnable setup commands.

Validation:

• python3 -m py_compile sentiment_app.py
• git diff --check
• npx --yes markdown-link-check README.md --quiet