fix: restore polished account experience and align AI with Lark design

backend/app/schemas.py

@@ -232,6 +232,13 @@ class JobOutput(BaseModel):
     updated_at: datetime
 
 
+class PlatformSessionOutput(BaseModel):
+    authenticated: bool
+    expires_at: datetime | None
+    remaining_seconds: int | None
+    total_seconds: float | None
+
+
 class AccountOutput(BaseModel):
     email: str | None
     configured: bool
@@ -245,6 +252,7 @@ class AccountOutput(BaseModel):
     theme: Literal["light", "dark"]
     timezone: str
     page_size: int
+    session: PlatformSessionOutput
 
 
 class SessionOutput(BaseModel):