Only allow Tauri imports in the desktop app (#5995)

# Description of Changes
Adds an eslint rule to disallow importing any Tauri APIs outside the
desktop folder to help hint to developers that they should be following
the frontend architecture.

While doing this, I also discovered that you can provide a custom
message in the `no-restricted-imports` rule, which is nicer than the
comments that I'd previously added to the eslint config file to explain
why they weren't allowed:

```text
/Users/jamesbrunton/Dev/spdf1/frontend/src/core/components/shared/config/configSections/GeneralSection.tsx
  19:1  error  'src/core/contexts/PreferencesContext' import is restricted from being used by a pattern. Use @app/* imports instead of absolute src/ imports              no-restricted-imports
  20:1  error  '../../../../../core/contexts/AppConfigContext' import is restricted from being used by a pattern. Use @app/* imports instead of relative imports          no-restricted-imports
  21:1  error  '@tauri-apps/core' import is restricted from being used by a pattern. Tauri APIs are desktop-only. Review frontend/DeveloperGuide.md for structure advice  no-restricted-imports
```

frontend/eslint.config.mjs

@@ -13,6 +13,11 @@ const nodeGlobs = [
   '*.config.{js,ts,mjs}',
 ];
 
+const baseRestrictedImportPatterns = [
+  { regex: '^\\.', message: "Use @app/* imports instead of relative imports." },
+  { regex: '^src/', message: "Use @app/* imports instead of absolute src/ imports." },
+];
+
 export default defineConfig(
   {
     // Everything that contains 3rd party code that we don't want to lint
@@ -30,10 +35,7 @@ export default defineConfig(
       'no-restricted-imports': [
         'error',
         {
-          patterns: [
-            ".*", // Disallow any relative imports (they should be '@app/x/y/z' or similar)
-            "src/*", // Disallow any absolute imports (they should be '@app/x/y/z' or similar)
-          ],
+          patterns: baseRestrictedImportPatterns,
         },
       ],
       '@typescript-eslint/no-empty-object-type': [
@@ -59,6 +61,26 @@ export default defineConfig(
       ],
     },
   },
+  // Desktop-only packages must not be imported from core or proprietary code.
+  // Use the stub/shadow pattern instead: define a stub in src/core/ and override in src/desktop/.
+  {
+    files: srcGlobs,
+    ignores: ['src/desktop/**'],
+    rules: {
+      'no-restricted-imports': [
+        'error',
+        {
+          patterns: [
+            ...baseRestrictedImportPatterns,
+            {
+              regex: '^@tauri-apps/',
+              message: "Tauri APIs are desktop-only. Review frontend/DeveloperGuide.md for structure advice.",
+            },
+          ],
+        },
+      ],
+    },
+  },
   // Folders that have been cleaned up and are now conformant - stricter rules enforced here
   {
     files: ['src/saas/**/*.{js,mjs,jsx,ts,tsx}'],