Cursor-based pagination for /notifications

Switches `listForUser` from page-offset to cursor (`before` param,
opaque base64 of `{ts, id}`) ordered by `(created_at DESC, id DESC)`
for stable pagination even with simultaneous fan-out inserts.

Returns `{ rows, nextCursor }` instead of a bare array. Loader surfaces
`?before=<cursor>` on a "Load older" link at the bottom of the list,
shown only while `nextCursor !== null`. Default page size 50, capped
at 100. Malformed cursors fall back to "start from top" rather than
400ing — opaque cursors should not be a client validation surface.

Spec drift: delta spec adds three pagination scenarios (cursor pages
forward, tie-stable on identical `created_at`, malformed-cursor
graceful fallback). Design doc gets a new decision section explaining
the cursor choice over page-offset and why we don't compute totals.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/i18n/src/locales/de.ts

@@ -245,6 +245,7 @@ export default {
       title: "Benachrichtigungen",
       empty: "Noch keine Benachrichtigungen.",
       markAllRead: "Alle als gelesen markieren",
+      loadOlder: "Ältere laden",
       someone: "Jemand",
       summary: {
         followRequestReceived: "{{name}} möchte dir folgen",

packages/i18n/src/locales/en.ts

@@ -245,6 +245,7 @@ export default {
       title: "Notifications",
       empty: "No notifications yet.",
       markAllRead: "Mark all read",
+      loadOlder: "Load older",
       someone: "Someone",
       summary: {
         followRequestReceived: "{{name}} requested to follow you",