Treat .md like .txt; remove markus suggestion

Merge the .md extension case into the plain-text branch so it suggests ['editor', 'code'] instead of ['markus', 'editor', 'code']. Update the related test to use 'viewer'/'png' as the built-in guard example since 'markus'/'md' no longer applies.

src/backend/services/apps/SuggestedAppsService.test.ts

@@ -285,10 +285,12 @@ describe('SuggestedAppsService hosted-backing guard', () => {
         const { userId } = await makeUser();
         const sub = uniqueName('builtinlive');
         await server.stores.subdomain.create({ userId, subdomain: sub });
-        // 'markus' is the first built-in opener mapped to `.md`.
-        await pointBuiltinAt('markus', userId, hostedUrl(sub));
+        // 'viewer' is the first built-in opener mapped to `.png`.
+        await pointBuiltinAt('viewer', userId, hostedUrl(sub));
 
-        expect((await suggestFor('md')).map((a) => a.name)).toContain('markus');
+        expect((await suggestFor('png')).map((a) => a.name)).toContain(
+            'viewer',
+        );
     });
 
     it('drops a built-in-name opener whose hosted backing is gone', async () => {

src/backend/services/apps/SuggestedAppsService.ts

@@ -156,12 +156,9 @@ function suggestionsForExtension(ext: string): {
     if (CODE_EXTS.has(lower)) {
         return { names: ['code', 'editor'], isFallback: false };
     }
-    if (lower === 'txt' || lower === '') {
+    if (lower === 'txt' || lower === 'md' || lower === '') {
         return { names: ['editor', 'code'], isFallback: false };
     }
-    if (lower === 'md') {
-        return { names: ['markus', 'editor', 'code'], isFallback: false };
-    }
     if (IMAGE_EXTS.has(lower)) {
         return { names: ['viewer', 'draw'], isFallback: false };
     }