batch4: chats-in-sessions, force-send, /compact, right-rail file browser

Session 1:N Chat data model with backfill. Workspace switches to client-side
multi-tab pane management. Right-rail file browser with float-over viewer and
click-drag line selection replaces FileBrowserPane. Adds /compact streaming
summarizer (respects compact markers in context builder), force-send (cancels
in-flight, persists partial as 'cancelled', awaits cancellation completion via
deferred Promise + 5s timeout), message queue, stop generation, chat
auto-rename, session archive/unarchive with Closed Sessions section on repo
landing page. CHECK constraints on sessions.status, messages.role,
messages.status with KEEP IN SYNC comments tying to MESSAGE_ROLES /
MESSAGE_STATUSES const arrays. Deletes dead pane routes/hook and the
api.panes.* client block.

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

apps/web/src/components/AttachmentPreviewModal.tsx

@@ -0,0 +1,37 @@
+import type { Attachment } from '@/lib/attachments';
+import { CodeBlock } from '@/components/CodeBlock';
+import {
+  Dialog,
+  DialogContent,
+  DialogHeader,
+  DialogTitle,
+} from '@/components/ui/dialog';
+
+interface Props {
+  attachment: Attachment | null;
+  onClose: () => void;
+}
+
+export function AttachmentPreviewModal({ attachment, onClose }: Props) {
+  const title = attachment
+    ? attachment.kind === 'lines' && attachment.range
+      ? `${attachment.filename}:${attachment.range[0]}-${attachment.range[1]}`
+      : attachment.filename
+    : '';
+
+  return (
+    <Dialog open={attachment !== null} onOpenChange={() => onClose()}>
+      <DialogContent className="sm:max-w-2xl max-h-[80vh] overflow-y-auto">
+        <DialogHeader>
+          <DialogTitle className="font-mono text-sm">{title}</DialogTitle>
+        </DialogHeader>
+        {attachment && (
+          <CodeBlock
+            code={attachment.content}
+            lang={attachment.language ?? undefined}
+          />
+        )}
+      </DialogContent>
+    </Dialog>
+  );
+}