fix: restore correct file permissions, stop tracking .DS_Store

Almost every tracked file in the repo had drifted to mode 777 on disk
(only files created fresh this session were unaffected), and a chunk of
that drift had already been committed as spurious +x bits on plain
YAML/Markdown files (authentik.yaml, kustomization.yaml, coturn.yaml,
element-server-suite.yaml, TASKS.md, install.md, etc.) - none of these
need to be executable. Restored to 644 for regular files, 755 only for
actual scripts (postCreateCommand.sh, docker-init.sh, install-hooks.sh,
pre-commit hook, element-setup-linux.sh).

Also found element-setup-macos.command was missing +x despite having a
shebang and being meant for double-click execution on macOS - fixed.

Added .gitignore for .DS_Store and .claude/ and stopped tracking the five
.DS_Store files that had been committed by accident.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,2 @@
+.DS_Store
+.claude/