Improve landing page.

It's possible to select which transactions to show

src/allmende_payment_system/api/__init__.py

@@ -9,12 +9,25 @@ templates = get_jinja_renderer()
 
 
 @root_router.get("/")
-async def landing_page(request: Request, user: UserDep, session: SessionDep):
+async def landing_page(
+    request: Request, user: UserDep, session: SessionDep, account_id: int | None = None
+):
+    focused_account = None
     transactions = []
     for account in user.accounts:
+        if account_id is not None and account.id != account_id:
+            continue
+        if account_id is not None and account.id == account_id:
+            focused_account = account
         transactions += account.transactions
+
     transactions = sorted(transactions, key=lambda t: t.timestamp)
     return templates.TemplateResponse(
         "index.html.jinja",
-        context={"request": request, "user": user, "transactions": transactions},
+        context={
+            "request": request,
+            "user": user,
+            "transactions": transactions,
+            "focused_account": focused_account,
+        },
     )