chore: normalize line endings to LF via .gitattributes

Root-cause fix for mixed CRLF/LF in the repo (no .gitattributes previously).
Adds '* text=auto eol=lf' and explicit binary rules so all future checkouts
are LF. Renormalizes all tracked text files (no logical content change).
Backup tags: image qihuaiflow:backup-lineend-20260725-121632, git backup-lineend-pre-20260725-121632.

.gitattributes

@@ -0,0 +1,33 @@
+# Normalize all text files to LF on checkout/commit (root-cause fix for mixed line endings)
+* text=auto eol=lf
+
+# Binary assets: never normalize line endings or diff as text
+*.png binary
+*.jpg binary
+*.jpeg binary
+*.gif binary
+*.ico binary
+*.webp binary
+*.woff binary
+*.woff2 binary
+*.ttf binary
+*.eot binary
+*.otf binary
+*.pdf binary
+*.zip binary
+*.tar binary
+*.gz binary
+*.bz2 binary
+*.7z binary
+*.pyc binary
+*.so binary
+*.dll binary
+*.exe binary
+*.db binary
+*.sqlite binary
+*.sqlite3 binary
+*.wasm binary
+*.mp4 binary
+*.mp3 binary
+*.avi binary
+*.mov binary