feat(pds): add northsky, selfhosted.social and selfhosted fallback (#94)

Add Northsky and selfhosted.social as known PDS providers.
Change mapPdsHostToProvider to return { name: 'selfhosted', host }
for any resolved PDS host that doesn't match a known provider,
instead of returning null.

src/lib/pds-provider.ts

···
   { pattern: /^blacksky\.app$/i, name: 'blacksky' },
   { pattern: /\.eurosky\.social$/i, name: 'eurosky' },
   { pattern: /^eurosky\.social$/i, name: 'eurosky' },
+  { pattern: /\.northsky\.social$/i, name: 'northsky' },
+  { pattern: /^northsky\.social$/i, name: 'northsky' },
+  { pattern: /\.selfhosted\.social$/i, name: 'selfhosted-social' },
+  { pattern: /^selfhosted\.social$/i, name: 'selfhosted-social' },
 ];
 
-export function mapPdsHostToProvider(host: string): PdsProviderInfo | null {
+export function mapPdsHostToProvider(host: string): PdsProviderInfo {
   for (const entry of KNOWN_PDS_HOSTS) {
     if (entry.pattern.test(host)) {
       return { name: entry.name, host };
     }
   }
-  return null;
+  return { name: 'selfhosted', host };
 }
 
 export function extractPdsHostFromEndpoint(endpoint: string): string | null {

tests/lib/pds-provider.test.ts

···
     expect(result).toEqual({ name: 'eurosky', host: 'pds.eurosky.social' });
   });
 
-  it('returns null for unknown PDS host', () => {
-    expect(mapPdsHostToProvider('my-pds.example.com')).toBeNull();
+  it('maps northsky.social to northsky', () => {
+    const result = mapPdsHostToProvider('pds.northsky.social');
+    expect(result).toEqual({ name: 'northsky', host: 'pds.northsky.social' });
   });
 
-  it('returns null for self-hosted PDS', () => {
-    expect(mapPdsHostToProvider('pds.alice.dev')).toBeNull();
+  it('maps selfhosted.social to selfhosted-social', () => {
+    const result = mapPdsHostToProvider('pds.selfhosted.social');
+    expect(result).toEqual({ name: 'selfhosted-social', host: 'pds.selfhosted.social' });
+  });
+
+  it('returns selfhosted for unknown PDS host', () => {
+    expect(mapPdsHostToProvider('my-pds.example.com')).toEqual({
+      name: 'selfhosted',
+      host: 'my-pds.example.com',
+    });
+  });
+
+  it('returns selfhosted for individual self-hosted PDS', () => {
+    expect(mapPdsHostToProvider('pds.alice.dev')).toEqual({
+      name: 'selfhosted',
+      host: 'pds.alice.dev',
+    });
   });
 });