Let users see and unsubscribe their own push devices

Fixes duplicate/triple OS notifications on machines where more than
one browser (e.g. both Chrome and Edge on Windows) independently got
notification permission for the site — each held its own valid
subscription, so every event pushed to both.

Adds a userAgent column (captured client-side at subscribe time,
display-only) and a Settings > Account section listing a user's own
subscribed browsers with a friendly label plus an unsubscribe button,
backed by new GET/DELETE /api/push/subscriptions endpoints scoped to
the caller's own rows.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gh2UXUQUVBWroWEnn1FLFG

src/lib/push/client.ts

@@ -57,7 +57,7 @@ export async function ensurePushSubscribed(): Promise<void> {
     const saveRes = await fetch("/api/push/subscribe", {
       method: "POST",
       headers: { "Content-Type": "application/json" },
-      body: JSON.stringify(subscription.toJSON()),
+      body: JSON.stringify({ ...subscription.toJSON(), userAgent: navigator.userAgent }),
     });
     if (saveRes.ok) {
       console.log("[push] subscription saved server-side — done");