feat: Improve email content extraction by enhancing header stripping and cleaning logic

internal/ai/ai.go

@@ -94,7 +94,7 @@ While the 'Email Body' provides the question, your answer should be synthesized
 
 Instructions:
 - Language: Your response must be entirely in %s, regardless of the language used in the email content or context.
-- Format: Your reply must be raw HTML. Use appropriate HTML tags for structure and styling.
+- Format: CRITICAL: Your reply MUST be raw HTML. Use appropriate HTML tags for structure and styling. For example, wrap paragraphs in <p>...</p> tags and use <br> for line breaks if needed within a paragraph. Even a short sentence must be wrapped in HTML (e.g., <p>Yes.</p>).
 - Markdown: Do NOT wrap the HTML in markdown code blocks (e.g., %s).
 - Extraneous Text: Do not include a subject line. Do not include explanations, commentary, or any extra text that is not part of the direct answer.
 - Closing: Avoid generic closing statements like "If you have further questions...". Focus solely on answering the email.