Add .editorconfig to enforce tab-based indentation

[omghante]

Problem

• Project uses tabs for Java files (enforced by Spotless + formatter.xml)
• No .editorconfig file exists in the repo
• Contributors using editors that default to spaces get wrong indentation
• Causes Spotless check failures or unexpected formatting diffs

Proposed Changes

• Add .editorconfig at the repo root with:
  • Tab indentation for Java files
  • Space indentation for XML and YAML files
  • UTF-8 charset and LF line endings
  • Final newline insertion
• Settings match the existing Spotless and formatter.xml config