Add prompt to make Stirling your default PDF app (#4890)

# Description of Changes
- Adds a reusable banner component/system to the core app
- Adds banner at the top of the desktop app if Stirling isn't your
default PDF editor, with a button to make it your default
- Adds a permanent button in the settings to do it manually (in case
you've dismissed the banner)
- Simplifies the file loading logic to fix a bug where the input file
could be duplicated occasionally. Now, the TS just receives files from
one buffer, regardless of how they've been passed to the app in Rust.

## Caveats
I've only been able to get the setting of default apps working properly
on Mac. The Windows build isn't signed (yet) so we can't use the proper
API for it, so currently it just sends you to the Settings UI. I've also
not been able to test it on Linux at all.

frontend/src/desktop/components/shared/DefaultAppBanner.tsx

@@ -0,0 +1,27 @@
+import React, { useState } from 'react';
+import { useTranslation } from 'react-i18next';
+import { InfoBanner } from '@app/components/shared/InfoBanner';
+import { useDefaultApp } from '@app/hooks/useDefaultApp';
+
+export const DefaultAppBanner: React.FC = () => {
+  const { t } = useTranslation();
+  const { isDefault, isLoading, handleSetDefault } = useDefaultApp();
+  const [dismissed, setDismissed] = useState(false);
+
+  const handleDismissPrompt = () => {
+    setDismissed(true);
+  };
+
+  return (
+    <InfoBanner
+      icon="picture-as-pdf-rounded"
+      message={t('defaultApp.prompt.message', 'Make Stirling PDF your default application for opening PDF files.')}
+      buttonText={t('defaultApp.setDefault', 'Set Default')}
+      buttonIcon="check-circle-rounded"
+      onButtonClick={handleSetDefault}
+      onDismiss={handleDismissPrompt}
+      loading={isLoading}
+      show={!dismissed && isDefault === false}
+    />
+  );
+};

frontend/src/desktop/components/shared/config/configSections/DefaultAppSettings.tsx

@@ -0,0 +1,40 @@
+import React from 'react';
+import { Paper, Text, Button, Group } from '@mantine/core';
+import { useTranslation } from 'react-i18next';
+import { useDefaultApp } from '@app/hooks/useDefaultApp';
+
+export const DefaultAppSettings: React.FC = () => {
+  const { t } = useTranslation();
+  const { isDefault, isLoading, handleSetDefault } = useDefaultApp();
+
+  return (
+    <Paper withBorder p="md" radius="md">
+      <Group justify="space-between" align="center">
+        <div>
+          <Text fw={500} size="sm">
+            {t('settings.general.defaultPdfEditor', 'Default PDF editor')}
+          </Text>
+          <Text size="xs" c="dimmed" mt={4}>
+            {isDefault === true
+              ? t('settings.general.defaultPdfEditorActive', 'Stirling PDF is your default PDF editor')
+              : isDefault === false
+              ? t('settings.general.defaultPdfEditorInactive', 'Another application is set as default')
+              : t('settings.general.defaultPdfEditorChecking', 'Checking...')}
+          </Text>
+        </div>
+        <Button
+          variant={isDefault ? 'light' : 'filled'}
+          color="blue"
+          size="sm"
+          onClick={handleSetDefault}
+          loading={isLoading}
+          disabled={isDefault === true}
+        >
+          {isDefault
+            ? t('settings.general.defaultPdfEditorSet', 'Already Default')
+            : t('settings.general.setAsDefault', 'Set as Default')}
+        </Button>
+      </Group>
+    </Paper>
+  );
+};

frontend/src/desktop/components/shared/config/configSections/GeneralSection.tsx

@@ -0,0 +1,18 @@
+import React from 'react';
+import { Stack } from '@mantine/core';
+import CoreGeneralSection from '@core/components/shared/config/configSections/GeneralSection';
+import { DefaultAppSettings } from '@app/components/shared/config/configSections/DefaultAppSettings';
+
+/**
+ * Desktop extension of GeneralSection that adds default PDF editor settings
+ */
+const GeneralSection: React.FC = () => {
+  return (
+    <Stack gap="lg">
+      <DefaultAppSettings />
+      <CoreGeneralSection />
+    </Stack>
+  );
+};
+
+export default GeneralSection;