Improve iframe document CSS: serif font, normalize bold

- Switch to Georgia to better match Word's serif rendering
- Explicit font-weight:normal on body/p so mammoth's <strong> wrapping
  doesn't bleed into paragraph text
- Tighter heading/paragraph spacing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server.js

@@ -1554,9 +1554,19 @@ app.post('/api/admin/upload-attachment', requirePin, async (req, res) => {
     const { value: bodyHtml } = await mammoth.convertToHtml({ buffer });
     const htmlFilename = `${uuid}.html`;
     const page = `<!doctype html><html><head><meta charset="utf-8">
-<style>*{box-sizing:border-box}body{font-family:sans-serif;margin:0;padding:1rem;line-height:1.6;font-size:15px}
-img{max-width:100%}table{border-collapse:collapse;width:100%}td,th{border:1px solid #ccc;padding:.4em .6em}
-h1,h2,h3{margin-top:1em}</style>
+<style>
+*{box-sizing:border-box}
+body{font-family:Georgia,serif;margin:0;padding:1rem 1.5rem;line-height:1.7;font-size:15px;font-weight:normal;color:#111}
+p{margin:.6em 0;font-weight:normal}
+strong,b{font-weight:bold}
+h1{font-size:1.5em;margin:1em 0 .3em}
+h2{font-size:1.25em;margin:.9em 0 .3em}
+h3{font-size:1.1em;margin:.8em 0 .2em}
+img{max-width:100%}
+table{border-collapse:collapse;width:100%}
+td,th{border:1px solid #ccc;padding:.4em .6em}
+ul,ol{padding-left:1.5em}
+</style>
 </head><body>${bodyHtml}</body></html>`;
     await writeFile(join(ATTACHMENTS_DIR, htmlFilename), page);
     htmlUrl = `/api/attachments/${htmlFilename}`;