expense list: clickable month pill + fix scroll-to-month

- make the floating month pill interactive (button role, hover,
  focus ring) and jump to the active month's header on click/Enter
- scroll via a non-sticky month anchor + window.scrollTo instead of
  scrollIntoView, which no-ops on sticky headers that are always
  already in view

src/Expense/ExpenseList.tsx

@@ -8,6 +8,7 @@ import {
 } from "@mui/material";
 import { alpha } from "@mui/material/styles";
 import ExpandMoreIcon from "@mui/icons-material/ExpandMore";
+import KeyboardArrowUpIcon from "@mui/icons-material/KeyboardArrowUp";
 import { ListCellRenderer, CurrencyField, applyDisplayFormat, formatCurrency } from "../../react-openapi";
 import type { ExpenseItem, ExpenseFieldConfigs } from "./types";
 import { isExpense, monthKey, monthLabel, parseOccurredAt } from "./types";
@@ -139,6 +140,27 @@ export function ExpenseList({ items, fields }: ExpenseListProps) {
     setExpandedId((prev) => (prev === id ? null : id));
   }, []);
 
+  const jumpToActiveMonth = useCallback(() => {
+    if (!activeMonth) return;
+    const anchor = listRef.current?.querySelector<HTMLElement>(
+      `[data-month-anchor="${activeMonth}"]`,
+    );
+    if (!anchor) return;
+    const offset = window.matchMedia("(min-width: 900px)").matches ? 64 : 56;
+    const top = anchor.getBoundingClientRect().top + window.scrollY - offset;
+    window.scrollTo({ top, behavior: "smooth" });
+  }, [activeMonth]);
+
+  const handlePillKeyDown = useCallback(
+    (event: React.KeyboardEvent) => {
+      if (event.key === "Enter" || event.key === " ") {
+        event.preventDefault();
+        jumpToActiveMonth();
+      }
+    },
+    [jumpToActiveMonth],
+  );
+
   useEffect(() => {
     const root = listRef.current;
     if (!root || groups.length === 0) return;
@@ -179,6 +201,7 @@ export function ExpenseList({ items, fields }: ExpenseListProps) {
       <Box ref={listRef} sx={{ display: "flex", flexDirection: "column", gap: 4 }}>
         {groups.map((group) => (
           <Box key={group.key}>
+            <span data-month-anchor={group.key} aria-hidden="true" style={{ display: "block", height: 0 }} />
             <Box
               data-month-header={group.key}
               sx={{
@@ -237,25 +260,42 @@ export function ExpenseList({ items, fields }: ExpenseListProps) {
           left: "50%",
           transform: "translateX(-50%)",
           zIndex: 3,
-          pointerEvents: "none",
+          pointerEvents: activeMonth ? "auto" : "none",
           opacity: activeMonth ? 1 : 0,
           transition: "opacity 160ms ease",
         }}
       >
         <Box
+          role="button"
+          tabIndex={0}
+          onClick={jumpToActiveMonth}
+          onKeyDown={handlePillKeyDown}
           sx={{
             display: "flex",
             alignItems: "center",
+            gap: 0.5,
             px: 1.5,
             py: 0.75,
             borderRadius: "999px",
+            cursor: "pointer",
+            userSelect: "none",
             backgroundColor: (theme) => alpha(theme.palette.background.default, 0.85),
             backdropFilter: "blur(8px)",
             border: "1px solid",
             borderColor: "divider",
             boxShadow: 1,
+            transition: "background-color 160ms ease, transform 160ms ease",
+            "&:hover": {
+              backgroundColor: (theme) => alpha(theme.palette.background.default, 0.95),
+              transform: "translateY(-1px)",
+            },
+            "&:focus-visible": {
+              outline: "2px solid",
+              outlineColor: "primary.main",
+            },
           }}
         >
+          <KeyboardArrowUpIcon sx={{ fontSize: 14, color: "text.secondary" }} />
           <Typography variant="caption" fontWeight={700} color="text.secondary">
             {activeMonth ? monthLabel(activeMonth) : ""}
           </Typography>