chore: remove main.pdf from tracking, ignore all paper/*.pdf

PDF triggers git-lfs lock verification on the Gitea server.
LaTeX source in paper/sections/ is sufficient for version control.

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

.gitignore

@@ -52,7 +52,7 @@ Thumbs.db
 .env
 *.env
 
-# === LaTeX 编译产物 ===
+# === LaTeX 编译产物 & PDF ===
 paper/*.aux
 paper/*.bbl
 paper/*.blg
@@ -62,3 +62,4 @@ paper/*.out
 paper/*.fls
 paper/*.fdb_latexmk
 paper/*.synctex.gz
+paper/*.pdf