Fix SVG coordinate formatting broken on non-English JVM locales (Czech, German, etc.)

[tombarys]

Hi, this is my third PR in my whole life, so I hope it will work. I found a small bug leading to something like this:

I cooperated with Claude and here is the solution.

plotje version: 0.2.0

Description:
Charts render nearly empty when the JVM runs under a locale that uses a comma as the decimal separator (e.g. cs_CZ, de_DE).

Root cause:
scicloj/plotje/render/svg.clj formats SVG coordinates using (format "%.2f" v), which is locale-sensitive. On Czech locale this produces "12,00" instead of "12.00", turning a valid translate(12.00,181.00) into a malformed translate(12,00,181,00) with four arguments — breaking all point and axis placement.

Minimal repro:

;; Start the JVM with Czech locale, e.g.: -Duser.language=cs -Duser.country=CZ
(require '[scicloj.metamorph.ml.rdatasets :as rdatasets]
         '[scicloj.plotje.api :as pj])

(-> (rdatasets/datasets-iris)
    (pj/lay-point :sepal-length :sepal-width))
;; => chart appears nearly empty; SVG contains translate(5,10,3,50) etc.

Workaround:

(java.util.Locale/setDefault java.util.Locale/ROOT)

or add to deps.edn:

:jvm-opts ["-Duser.language=en" "-Duser.country=US"]

Suggested fix sent in this PR:
In svg.clj, the fmt helper has to be changed to pin the locale explicitly:

(defn- fmt [v]
  (if v (String/format java.util.Locale/ROOT "%.2f" (to-array [(double v)])) "0.00"))

Best.