refactor(desktop): drop per-session icons, read-only cross-profile reads

The per-session icon picker added more noise than value — rip it out end
to end (sessions.icon column, set_session_icon, the PATCH field, the
picker UI, and the SessionInfo.icon type).

The cross-profile session aggregator now opens each profile's state.db
read-only (mode=ro, no schema init), so listing other profiles on every
sidebar refresh never DDLs or takes a write lock on their live DBs. The
single-profile hot path stays on par with /api/sessions.

tests/hermes_cli/test_web_server.py

@@ -381,36 +381,6 @@ class TestWebServerEndpoints:
         resp = self.client.patch("/api/sessions/no-fields", json={})
         assert resp.status_code == 400
 
-    def test_set_and_clear_session_icon_via_patch(self):
-        """PATCH icon sets a per-session glyph; "" clears it back to None."""
-        from hermes_state import SessionDB
-
-        db = SessionDB()
-        try:
-            db.create_session(session_id="icon-me", source="cli")
-        finally:
-            db.close()
-
-        resp = self.client.patch("/api/sessions/icon-me", json={"icon": "🦊"})
-        assert resp.status_code == 200
-        assert resp.json()["icon"] == "🦊"
-
-        db = SessionDB()
-        try:
-            assert db.get_session("icon-me")["icon"] == "🦊"
-        finally:
-            db.close()
-
-        resp = self.client.patch("/api/sessions/icon-me", json={"icon": ""})
-        assert resp.status_code == 200
-        assert resp.json()["icon"] is None
-
-        db = SessionDB()
-        try:
-            assert db.get_session("icon-me")["icon"] is None
-        finally:
-            db.close()
-
     def test_profiles_sessions_tags_default_profile(self):
         """The cross-profile aggregator returns the default profile's rows
         tagged profile="default" (single-profile parity with /api/sessions)."""