fix(client): update sass functions to use new module syntax

[mapxn]

• Replace str-index with string.index
• Replace str-slice with string.slice
• Replace str-length with string.length

These changes address deprecation warnings from Dart Sass 3.0.0

[mapxn]

Fix the following warnings:

Deprecation Warning: Global built-in functions are deprecated and will be removed in Dart Sass 3.0.0.
Use string.index instead.

More info and automated migrator: https://sass-lang.com/d/import

   ╷
24 │   @if not str-index($svg, xmlns) {
   │           ^^^^^^^^^^^^^^^^^^^^^^
   ╵
    src/styles/helpers/_svg.scss 24:11  svg-url()
    src/styles/helpers/_svg.scss 48:21  background-svg()
    src/styles/meta.scss 31:5           root stylesheet

Deprecation Warning: Global built-in functions are deprecated and will be removed in Dart Sass 3.0.0.
Use string.slice instead.

More info and automated migrator: https://sass-lang.com/d/import

  ╷
8 │     str-slice($string, 1, $index - 1) + $replace +
  │     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  ╵
    src/styles/helpers/_svg.scss 8:5    string-replace()
    src/styles/helpers/_svg.scss 25:11  svg-url()
    src/styles/helpers/_svg.scss 48:21  background-svg()
    src/styles/meta.scss 31:5           root stylesheet

Deprecation Warning: Global built-in functions are deprecated and will be removed in Dart Sass 3.0.0.
Use string.length instead.

More info and automated migrator: https://sass-lang.com/d/import

   ╷
10 │         str-slice($string, $index + str-length($search)),
   │                                     ^^^^^^^^^^^^^^^^^^^
   ╵
    src/styles/helpers/_svg.scss 10:37  string-replace()
    src/styles/helpers/_svg.scss 25:11  svg-url()
    src/styles/helpers/_svg.scss 48:21  background-svg()
    src/styles/meta.scss 31:5           root stylesheet

Deprecation Warning: Global built-in functions are deprecated and will be removed in Dart Sass 3.0.0.
Use string.slice instead.

More info and automated migrator: https://sass-lang.com/d/import

   ╷
10 │         str-slice($string, $index + str-length($search)),
   │         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   ╵
    src/styles/helpers/_svg.scss 10:9   string-replace()
    src/styles/helpers/_svg.scss 25:11  svg-url()
    src/styles/helpers/_svg.scss 48:21  background-svg()
    src/styles/meta.scss 31:5           root stylesheet