perf(server,web): cursor-paginate session history with tail-first loading (#202)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/web/src/lib/strings-en.ts

@@ -758,6 +758,12 @@ Scenarios:
     statusCompacting: "Compacting",
     pendingApprovals: (n: number) => `${n} pending approval${n > 1 ? "s" : ""}`,
     jumpToLatest: "Jump to latest",
+    /** Top-of-stream affordance while the previous history window is being fetched (scroll-up backfill). */
+    loadingEarlier: "Loading earlier messages…",
+    /** Top-of-stream affordance after a backfill failure: click to retry fetching the previous window. */
+    loadEarlierRetry: "Failed to load earlier messages — click to retry",
+    /** Top-of-stream marker once the loaded history reaches the very beginning (shown only after a backfill happened). */
+    historyBeginning: "Beginning of conversation",
     /** Conversation minimap (tick rail over the stream's left gutter): rail aria-label. */
     outlineTitle: "Outline",
     /** Tick accessible name: turn number + the question (or the no-text placeholder). */