Remove ambiguous translations and fix invalid translations (#4841)

# Description of Changes
`i18next` allows this pattern for translations, which we use quite a few
times in our current translation files:

```json
{
  "a": {
     "b": "hello"
   },
   "a.b": "world"
}
```

This makes it ambiguous when selecting `a.b` which string will be
retrieved. We have seen issues in other languages in the current release
like this:

<img width="325" height="249" alt="image"
src="https://github.com/user-attachments/assets/f24a29f0-550f-49b8-b355-c5e5eb436558"
/>

because we are expecting this:

<img width="1022" height="210" alt="image"
src="https://github.com/user-attachments/assets/b6d5cdd4-96cd-4b2b-8f1a-465da8bf70c8"
/>

but the Spanish file has:

<img width="312" height="136" alt="image"
src="https://github.com/user-attachments/assets/1e13392c-8484-47d1-b0c4-19d52b3ea5eb"
/>

and no `removeDigitalSignature` key on its own. 

This PR resolves all of these ambiguities in the source by restructuring
all of the keys to uniquely target either an object or a string, not
both. It also adds a test which will fail on any keys with a `.` in
their name, therefore making it impossible to add anything ambiguous.

frontend/src/core/tests/translationStructure.test.ts

@@ -0,0 +1,51 @@
+import { describe, test, expect } from 'vitest';
+import fs from 'fs';
+import path from 'path';
+
+const LOCALES_DIR = path.join(__dirname, '../../../public/locales');
+
+const getLocaleDirectories = () => {
+  if (!fs.existsSync(LOCALES_DIR)) {
+    return [];
+  }
+
+  return fs.readdirSync(LOCALES_DIR, { withFileTypes: true })
+    .filter(dirent => dirent.isDirectory())
+    .map(dirent => dirent.name);
+};
+
+const findDottedKeys = (node: unknown, segments: string[] = []): string[] => {
+  if (!node || typeof node !== 'object') {
+    return [];
+  }
+
+  const issues: string[] = [];
+  for (const [key, value] of Object.entries(node as Record<string, unknown>)) {
+    if (key.includes('.')) {
+      issues.push([...segments, key].join('.'));
+    }
+    issues.push(...findDottedKeys(value, [...segments, key]));
+  }
+  return issues;
+};
+
+const localeDirectories = getLocaleDirectories();
+
+describe('Translation key structure', () => {
+  test('should locate locales directory', () => {
+    expect(fs.existsSync(LOCALES_DIR)).toBe(true);
+  });
+
+  test('should have at least one locale directory', () => {
+    expect(localeDirectories.length).toBeGreaterThan(0);
+  });
+
+  test.each(localeDirectories)('should not contain dotted keys in %s/translation.json', (localeDir) => {
+    const translationFile = path.join(LOCALES_DIR, localeDir, 'translation.json');
+    expect(fs.existsSync(translationFile)).toBe(true);
+
+    const data = JSON.parse(fs.readFileSync(translationFile, 'utf8'));
+    const dottedKeys = findDottedKeys(data);
+    expect(dottedKeys, `Dotted keys found in ${localeDir}: ${dottedKeys.join(', ')}`).toHaveLength(0);
+  });
+});