Subscription upgrade/downgrade + tier management (#3927)

## Summary

- Tier upgrade/downgrade in the Store. The Subscriptions tab now shows
all tiers including the user's current one. Other tiers swap "Subscribe"
→ "Switch" when the user already has a sub, and clicking them calls the
new `POST /subscriptions/@me/change-tier` endpoint with a
direction-aware confirm (upgrade charges prorated diff now, downgrade
gives account credit).
- Owned-tier card renders a **Current Plan** badge in place of the
purchase button. Resolution logic in `resolveCosmetics` now reads
`userMeResponse.player.subscription.tier` (with flare fallback) and
marks that tier as `owned`.
- AccountModal's `<subscription-panel>` reworked into a proper
two-column layout:
- **Left**: tier name, `$X.XX/mo` price, description, daily Pu/Caps
amounts.
- **Right**: status badge (Active / Renews date / Cancels date),
`[Manage] [Change Tier]` button row, `[Cancel]` centered underneath.
When `cancelAtPeriodEnd === true`, the row collapses to a single
`[Reactivate]` button (opens the Stripe portal).
- New `<o-button size="xs">` variant (`py-2 px-3 text-xs`) for the
compact panel buttons.
- Store dollar-purchase price label now supports an optional suffix
(`/mo` for subs only) via a `priceSuffix` prop plumbed through
`CosmeticContainer` → `PurchaseButton`.
- `Api.ts` gains `changeSubscriptionTier(tierName)` with the same
401-handling pattern as the existing subscription helpers.


<img width="1114" height="728" alt="Screenshot 2026-05-14 at 7 09 20 PM"
src="https://github.com/user-attachments/assets/688f83d5-4010-4580-9214-6885af8ec98e"
/>

<img width="1038" height="276" alt="Screenshot 2026-05-14 at 7 09 33 PM"
src="https://github.com/user-attachments/assets/458197f5-a0d4-4c32-bc55-31e5679629b5"
/>

<img width="887" height="286" alt="Screenshot 2026-05-14 at 7 09 55 PM"
src="https://github.com/user-attachments/assets/8149ed82-89cc-4bbe-83de-3614f886b331"
/>

## Discord

evan

src/client/Api.ts

@@ -196,6 +196,40 @@ export async function cancelSubscription(): Promise<boolean> {
   }
 }
 
+export async function changeSubscriptionTier(
+  tierName: string,
+): Promise<boolean> {
+  try {
+    const response = await fetch(
+      `${getApiBase()}/subscriptions/@me/change-tier`,
+      {
+        method: "POST",
+        headers: {
+          "Content-Type": "application/json",
+          Authorization: await getAuthHeader(),
+        },
+        body: JSON.stringify({ tierName }),
+      },
+    );
+    if (response.status === 401) {
+      await logOut();
+      return false;
+    }
+    if (!response.ok) {
+      console.error(
+        "changeSubscriptionTier: request failed",
+        response.status,
+        response.statusText,
+      );
+      return false;
+    }
+    return true;
+  } catch (e) {
+    console.error("changeSubscriptionTier: request failed", e);
+    return false;
+  }
+}
+
 export async function openSubscriptionPortal(): Promise<string | false> {
   try {
     const response = await fetch(`${getApiBase()}/subscriptions/@me/portal`, {