perf(ui): harden theme-provider hydration seam + cache per-row Intl formatters

Theme provider now initializes to the SSR-stable default and reconciles
the stored preference in a mount effect instead of reading localStorage
in the useState initializer — closes the same React #418 hydration seam
the sidebar already fixed (the inline script still handles the pre-paint
class, so no FOUC). Add a process-wide Intl formatter cache and route the
per-row workout/medication list helpers through it so a formatter is
constructed once per (locale, options) rather than once per row per
render.

src/lib/intl/__tests__/formatter-cache.test.ts

@@ -0,0 +1,40 @@
+import { describe, it, expect } from "vitest";
+
+import { getNumberFormat, getDateTimeFormat } from "../formatter-cache";
+
+describe("formatter-cache", () => {
+  it("returns the same NumberFormat instance for identical (locale, options)", () => {
+    const a = getNumberFormat("en", { maximumFractionDigits: 2 });
+    const b = getNumberFormat("en", { maximumFractionDigits: 2 });
+    expect(a).toBe(b);
+  });
+
+  it("returns distinct NumberFormat instances for different options", () => {
+    const a = getNumberFormat("en", { maximumFractionDigits: 2 });
+    const b = getNumberFormat("en", { maximumFractionDigits: 0 });
+    expect(a).not.toBe(b);
+  });
+
+  it("returns distinct instances per locale", () => {
+    const a = getNumberFormat("en", { maximumFractionDigits: 1 });
+    const b = getNumberFormat("de", { maximumFractionDigits: 1 });
+    expect(a).not.toBe(b);
+  });
+
+  it("caches DateTimeFormat and still formats correctly", () => {
+    const a = getDateTimeFormat("en-CA", {
+      timeZone: "UTC",
+      year: "numeric",
+      month: "2-digit",
+      day: "2-digit",
+    });
+    const b = getDateTimeFormat("en-CA", {
+      timeZone: "UTC",
+      year: "numeric",
+      month: "2-digit",
+      day: "2-digit",
+    });
+    expect(a).toBe(b);
+    expect(a.format(new Date("2026-05-08T12:00:00Z"))).toBe("2026-05-08");
+  });
+});