feat(export): enhance export functionality with incremental updates and latest-day slice

- Updated README to clarify bulk export options, including incremental updates and latest-day slice.
- Modified CLI export flags to support `--since incremental` for exporting only new or changed sessions since the last export.
- Implemented logic in the export API to handle the new export options, including returning a 422 error when there is nothing to export.
- Enhanced session filtering for the latest activity day and improved project matching for exports.
- Added unit tests for new export features and validation of state management.

Author: leostar0412

README.md

@@ -19,13 +19,14 @@ Browse and export Claude Code chat history — Web GUI and CLI.
 - **Smooth transitions** — staggered card/message animations, crossfade content swaps
 - **Scroll-to-top button** in bottom-right corner
 - **Per-model badges** in session header
-- **Bulk export** — download all sessions as a zip
+- **Bulk export** — download all sessions, incremental updates, or latest-day slice as a zip; if there is nothing to export, the API returns **422** JSON (`Nothing to export`) instead of an empty zip
 
 ### CLI Export
 - Standalone script to export all sessions to Markdown with YAML frontmatter
 - Rich Markdown: token usage, tool calls, thinking blocks, model info, timestamps
-- `--since last` flag for incremental export (only new/updated sessions)
-- `--project` flag to export a specific project
+- `--since last` — export every session that overlaps the **latest UTC calendar day** present in your history (zip name includes `last-MM-DD`)
+- `--since incremental` — export only sessions **new or changed since the last export** (file mtime + saved state)
+- `--project` flag to export a subset of projects
 
 ## Quick Start
 
@@ -60,7 +61,7 @@ python app.py --base-dir /path/to/claude/projects
 ```bash
 # Activate venv first (see above), then:
 
-# List all projects (shows directory names you can use with --project)
+# List all projects (first column is a friendly name; --project accepts that or the dir slug)
 python scripts/export.py list
 
 # Export all sessions as zip
@@ -69,14 +70,17 @@ python scripts/export.py
 # Export to specific directory, no zip
 python scripts/export.py --out ./exports --no-zip
 
-# Incremental export (only new sessions since last run)
+# Latest calendar day (UTC): all sessions active on that day; zip like claude-code-export-last-04-06-2026-05-08.zip
 python scripts/export.py --since last
 
-# Export specific project only (substring match on directory name)
+# Incremental (only new/updated sessions since last run, using export state)
+python scripts/export.py --since incremental
+
+# Export specific project only (substring on friendly name from list and/or dir name under ~/.claude/projects/)
 python scripts/export.py --project boost-capy
 ```
 
-The `--project` flag matches against the directory names under `~/.claude/projects/`. These are path-based names like `F--boost-capy` or `d--harbor-forge`. You can use any substring — for example `boost-capy` will match `F--boost-capy`. Run `python scripts/export.py list` to see all available project names.
+The `--project` flag matches a **case-insensitive substring** of either the **Project** column from `list` (derived from the session working directory) or the internal directory name under `~/.claude/projects/` (for example `F--boost-capy` or `d--harbor-forge`). A substring like `boost-capy` matches `F--boost-capy`; you can also paste the friendly name shown in `list`.
 
 ## Data Source