fix: resolve Prism syntax highlighting error in peer config modal

The PeerViewModal and InterfaceViewModal components displayed:
  Error: The language "undefined" has no grammar.

Root cause: vue-prism-component bundles its own instance of prismjs
(separate from the one imported elsewhere), so registering the ini
grammar on a different Prism instance had no effect on the one
actually performing the highlighting.

Fix: bypass vue-prism-component entirely. Import a single Prism
instance via a new helper (frontend/src/helpers/prism-setup.js) that
also registers the ini grammar, then render highlighted HTML directly
with <pre><code v-html="highlightedConfig">. This ensures the same
Prism object that owns the ini grammar is the one calling highlight().

Affected files:
- frontend/src/helpers/prism-setup.js (new): imports prismjs, defines
  the ini grammar on the Prism instance, re-exports it
- frontend/src/components/PeerViewModal.vue: replaces <Prism> with
  v-html using the shared Prism instance
- frontend/src/components/InterfaceViewModal.vue: same replacement
- frontend/src/main.js: removes the now-unused prism-setup import

Fixes #726

frontend/src/helpers/prism-setup.js

@@ -0,0 +1,39 @@
+import Prism from 'prismjs'
+
+Prism.languages.ini = {
+  comment: {
+    pattern: /(^[ \f\t\v]*)[#;][^\n\r]*/m,
+    lookbehind: true
+  },
+  section: {
+    pattern: /(^[ \f\t\v]*)\[[^\n\r\]]*\]?/m,
+    lookbehind: true,
+    inside: {
+      'section-name': {
+        pattern: /(^\[[ \f\t\v]*)[^ \f\t\v\]]+(?:[ \f\t\v]+[^ \f\t\v\]]+)*/,
+        lookbehind: true,
+        alias: 'selector'
+      },
+      punctuation: /\[|\]/
+    }
+  },
+  key: {
+    pattern: /(^[ \f\t\v]*)[^ \f\n\r\t\v=]+(?:[ \f\t\v]+[^ \f\n\r\t\v=]+)*(?=[ \f\t\v]*=)/m,
+    lookbehind: true,
+    alias: 'attr-name'
+  },
+  value: {
+    pattern: /(=[ \f\t\v]*)[^ \f\n\r\t\v]+(?:[ \f\t\v]+[^ \f\n\r\t\v]+)*/,
+    lookbehind: true,
+    alias: 'attr-value',
+    inside: {
+      'inner-value': {
+        pattern: /^("|').+(?=\1$)/,
+        lookbehind: true
+      }
+    }
+  },
+  punctuation: /=/
+}
+
+export default Prism