Add React client: session browser, message viewer, filters, search, redaction, export

Full React 18 client application for interactive session browsing:

app.tsx:
- Root component orchestrating session list, viewer, filters, search,
  redaction controls, and export — wires together useSession and
  useFilters hooks
- Keyboard navigation: j/k or arrow keys for message focus, Escape to
  clear search and redaction selection, "/" to focus search input
- Derives filtered messages, match count, visible UUIDs, and category
  counts via useMemo to avoid render-time side effects

hooks/useSession.ts:
- Manages session list and detail fetching state (loading, error,
  data) with useCallback-wrapped fetch functions
- Auto-loads session list on mount

hooks/useFilters.ts:
- Category filter state with toggle, set-all, and preset support
- Text search with debounced query propagation
- Manual redaction workflow: select messages, confirm to move to
  redacted set, undo individual or all redactions, select-all-visible
- Auto-redact toggle for the sensitive-redactor module
- Returns memoized object to prevent unnecessary re-renders

components/SessionList.tsx:
- Two-phase navigation: project list → session list within a project
- Groups sessions by project, shows session count and latest modified
  date per project, auto-drills into the correct project when a session
  is selected externally
- Formats project directory names back to paths (leading dash → /)

components/SessionViewer.tsx:
- Renders filtered messages with redacted dividers inserted where
  manually redacted messages were removed from the visible sequence
- Loading spinner, empty state for no session / no filter matches
- Scrolls focused message into view via ref

components/MessageBubble.tsx:
- Renders individual messages with category-specific Tailwind border
  and background colors
- Markdown rendering via marked + highlight.js, with search term
  highlighting that splits HTML tags to avoid corrupting attributes
- Click-to-select for manual redaction, visual selection indicator
- Auto-redact mode applies sensitive-redactor to content before render
- dangerouslySetInnerHTML is safe here: content is from local
  user-owned JSONL files, not untrusted external input

components/FilterPanel.tsx:
- Checkbox list for all 9 message categories with auto-redact toggle

components/SearchBar.tsx:
- Debounced text input (200ms) with match count display
- "/" keyboard shortcut to focus, × button to clear

components/ExportButton.tsx:
- POSTs current session + visible/redacted UUIDs + auto-redact flag
  to /api/export, downloads the returned HTML blob as a file

components/RedactedDivider.tsx:
- Dashed-line visual separator indicating redacted content gap

lib/types.ts:
- Re-exports shared types via @shared path alias for client imports

lib/constants.ts:
- Tailwind CSS class mappings per message category (border + bg colors)

lib/markdown.ts:
- Configured marked + highlight.js instance with search highlighting
  that operates on text segments only (preserves HTML tags intact)

styles/main.css:
- Tailwind base/components/utilities, custom scrollbar, highlight.js
  overrides, search highlight mark, redaction selection outline,
  message dimming for non-matching search results

index.html + main.tsx:
- Vite entry point mounting React app into #root with StrictMode

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/client/components/ExportButton.tsx

@@ -0,0 +1,60 @@
+import React, { useState } from "react";
+import type { SessionDetailResponse } from "../lib/types";
+
+interface Props {
+  session: SessionDetailResponse;
+  visibleMessageUuids: string[];
+  redactedMessageUuids: string[];
+  autoRedactEnabled: boolean;
+}
+
+export function ExportButton({
+  session,
+  visibleMessageUuids,
+  redactedMessageUuids,
+  autoRedactEnabled,
+}: Props) {
+  const [exporting, setExporting] = useState(false);
+
+  async function handleExport() {
+    setExporting(true);
+    try {
+      const res = await fetch("/api/export", {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({
+          session,
+          visibleMessageUuids,
+          redactedMessageUuids,
+          autoRedactEnabled,
+        }),
+      });
+      if (!res.ok) throw new Error(`Export failed: HTTP ${res.status}`);
+
+      const blob = await res.blob();
+      const url = URL.createObjectURL(blob);
+      const a = document.createElement("a");
+      a.href = url;
+      a.download = `session-${session.id}.html`;
+      document.body.appendChild(a);
+      a.click();
+      document.body.removeChild(a);
+      URL.revokeObjectURL(url);
+    } catch (err) {
+      console.error("Export failed:", err);
+      alert("Export failed. Check console for details.");
+    } finally {
+      setExporting(false);
+    }
+  }
+
+  return (
+    <button
+      onClick={handleExport}
+      disabled={exporting}
+      className="px-4 py-1.5 text-sm bg-gray-900 text-white rounded-md hover:bg-gray-800 disabled:opacity-50 flex-shrink-0"
+    >
+      {exporting ? "Exporting..." : "Export HTML"}
+    </button>
+  );
+}