Add watermark tiling, rotation types, margin/scale controls

Wire tiled, margin, and scale UI controls to JobConfig and pass
through to WatermarkConfig. Add tiled text and image watermark
implementations that repeat across the full image. Add font family
filesystem search for named fonts. Add WatermarkRotation enum.

pixstrip-cli/src/main.rs

@@ -314,6 +314,9 @@ fn cmd_process(args: CmdProcessArgs) {
             opacity: args.watermark_opacity,
             color: [255, 255, 255, 255],
             font_family: None,
+            rotation: None,
+            tiled: false,
+            margin: 10,
         });
     }
     if args.rename_prefix.is_some() || args.rename_suffix.is_some() || args.rename_template.is_some() {