fix: replace deprecated imaging.quality with per-format quality keys

Hugo deprecated the top-level imaging.quality key in v0.163.0 (removed in
a future release). Move the value (75) to imaging.jpeg/webp/avif.quality,
preserving identical encoding behavior and silencing the deprecation
warning on build.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: amuraru

config/_default/config.toml

@@ -166,8 +166,15 @@ enableGitInfo = true
 # Image processing configuration.
 [imaging]
 resampleFilter = "CatmullRom"
-quality = 75
 anchor = "smart"
+# Per-format quality replaces the deprecated top-level imaging.quality
+# (deprecated in Hugo v0.163.0, removed in a future release).
+[imaging.jpeg]
+quality = 75
+[imaging.webp]
+quality = 75
+[imaging.avif]
+quality = 75
 
 [minify]
   minifyOutput = true