Add custom auth store directory

[liorb-mountapps]

Summary

• Add XURL_STORE_DIR to store .xurl in a custom directory.
• Import .twurlrc from the same custom directory when XURL_STORE_DIR is set.
• Create the store directory before saving tokens.
• Document container volume usage.

Tests

• gofmt on touched Go files
• go test -v ./...
• go build -v ./...
• git diff --check
• Manually tested OAuth with real X app credentials using XURL_STORE_DIR