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/components/tools/flatten/FlattenSettings.tsx

@@ -20,7 +20,7 @@ const FlattenSettings = ({ parameters, onParameterChange, disabled = false }: Fl
           disabled={disabled}
           label={
             <div>
-              <Text size="sm">{t('flatten.options.flattenOnlyForms', 'Flatten only forms')}</Text>
+              <Text size="sm">{t('flatten.options.flattenOnlyForms.label', 'Flatten only forms')}</Text>
               <Text size="xs" c="dimmed">
                 {t('flatten.options.flattenOnlyForms.desc', 'Only flatten form fields, leaving other interactive elements intact')}
               </Text>

frontend/src/core/components/tools/merge/MergeSettings.test.tsx

@@ -93,8 +93,8 @@ describe('MergeSettings', () => {
     );
 
     // Verify that translation keys are being called
-    expect(mockT).toHaveBeenCalledWith('merge.removeDigitalSignature', 'Remove digital signature in the merged file?');
-    expect(mockT).toHaveBeenCalledWith('merge.generateTableOfContents', 'Generate table of contents in the merged file?');
+    expect(mockT).toHaveBeenCalledWith('merge.removeDigitalSignature.label', 'Remove digital signature in the merged file?');
+    expect(mockT).toHaveBeenCalledWith('merge.generateTableOfContents.label', 'Generate table of contents in the merged file?');
   });
   
 });

frontend/src/core/components/tools/merge/MergeSettings.tsx

@@ -19,14 +19,14 @@ const MergeSettings: React.FC<MergeSettingsProps> = ({
   return (
     <Stack gap="md">
       <Checkbox
-        label={t('merge.removeDigitalSignature', 'Remove digital signature in the merged file?')}
+        label={t('merge.removeDigitalSignature.label', 'Remove digital signature in the merged file?')}
         checked={parameters.removeDigitalSignature}
         onChange={(event) => onParameterChange('removeDigitalSignature', event.currentTarget.checked)}
         disabled={disabled}
       />
 
       <Checkbox
-        label={t('merge.generateTableOfContents', 'Generate table of contents in the merged file?')}
+        label={t('merge.generateTableOfContents.label', 'Generate table of contents in the merged file?')}
         checked={parameters.generateTableOfContents}
         onChange={(event) => onParameterChange('generateTableOfContents', event.currentTarget.checked)}
         disabled={disabled}

frontend/src/core/components/tools/sanitize/SanitizeSettings.test.tsx

@@ -161,8 +161,8 @@ describe('SanitizeSettings', () => {
 
     // Verify that translation keys are being called (just check that it was called, not specific order)
     expect(mockT).toHaveBeenCalledWith('sanitize.options.title', expect.any(String));
-    expect(mockT).toHaveBeenCalledWith('sanitize.options.removeJavaScript', expect.any(String));
-    expect(mockT).toHaveBeenCalledWith('sanitize.options.removeEmbeddedFiles', expect.any(String));
+    expect(mockT).toHaveBeenCalledWith('sanitize.options.removeJavaScript.label', expect.any(String));
+    expect(mockT).toHaveBeenCalledWith('sanitize.options.removeEmbeddedFiles.label', expect.any(String));
     expect(mockT).toHaveBeenCalledWith('sanitize.options.note', expect.any(String));
   });
 

frontend/src/core/components/tools/sanitize/SanitizeSettings.tsx

@@ -12,8 +12,8 @@ const SanitizeSettings = ({ parameters, onParameterChange, disabled = false }: S
   const { t } = useTranslation();
 
   const options = (Object.keys(defaultParameters) as Array<keyof SanitizeParameters>).map((key) => ({
-    key: key,
-    label: t(`sanitize.options.${key}`, key),
+    key,
+    label: t(`sanitize.options.${key}.label`, key),
     description: t(`sanitize.options.${key}.desc`, `${key} from the PDF`),
     default: defaultParameters[key],
   }));