feat(federation): prune dmAlternatives on removeInstanceSpaces

Drops the given origin from every inner (origin→localId) map; removes
the outer federatedId entry when its inner map becomes empty. Keeps the
store from accumulating stale origin references across long sessions
with connect/disconnect churn.

packages/web/src/stores/spaceStore.dmAlternatives.test.ts

@@ -113,4 +113,46 @@ describe('spaceStore.dmAlternatives', () => {
     expect(byOrigin?.get('https://remote.example')).toBe('remote-new');
     expect(byOrigin?.size).toBe(1);
   });
+
+  it('removeInstanceSpaces drops the origin from every inner map', () => {
+    useSpaceStore.getState().populateFromReady(
+      '',
+      [],
+      [],
+      [makeDm('home-1', 'fed-aaa'), makeDm('home-2', 'fed-bbb')],
+      null,
+      0,
+    );
+    useSpaceStore.getState().populateFromReady(
+      'https://remote.example',
+      [],
+      [],
+      [makeDm('remote-1', 'fed-aaa'), makeDm('remote-2', 'fed-bbb')],
+      null,
+      0,
+    );
+
+    useSpaceStore.getState().removeInstanceSpaces('https://remote.example');
+
+    const alts = useSpaceStore.getState().dmAlternatives;
+    expect(alts.get('fed-aaa')?.has('https://remote.example')).toBe(false);
+    expect(alts.get('fed-aaa')?.get('')).toBe('home-1');
+    expect(alts.get('fed-bbb')?.has('https://remote.example')).toBe(false);
+    expect(alts.get('fed-bbb')?.get('')).toBe('home-2');
+  });
+
+  it('removeInstanceSpaces deletes federatedId entry if its inner map becomes empty', () => {
+    useSpaceStore.getState().populateFromReady(
+      'https://remote.example',
+      [],
+      [],
+      [makeDm('remote-only', 'fed-solo')],
+      null,
+      0,
+    );
+
+    useSpaceStore.getState().removeInstanceSpaces('https://remote.example');
+
+    expect(useSpaceStore.getState().dmAlternatives.has('fed-solo')).toBe(false);
+  });
 });

packages/web/src/stores/spaceStore.ts

@@ -870,6 +870,14 @@ export const useSpaceStore = create<SpaceState>((set, get) => ({
         }
       }
 
+      // Prune dmAlternatives: drop this origin from every inner map.
+      const dmAlternatives = new Map<string, Map<string, string>>();
+      for (const [fid, byOrigin] of state.dmAlternatives) {
+        const nextInner = new Map(byOrigin);
+        nextInner.delete(origin);
+        if (nextInner.size > 0) dmAlternatives.set(fid, nextInner);
+      }
+
       return {
         spaces: remainingSpaces,
         channelToSpaceMap,
@@ -877,6 +885,7 @@ export const useSpaceStore = create<SpaceState>((set, get) => ({
         channelPermissions,
         channelOriginMap,
         spacePermissions,
+        dmAlternatives,
         currentSpaceId: remainingSpaces.find(s => s.id === state.currentSpaceId)
           ? state.currentSpaceId
           : null,