v0.8.2: build fixes, WebUI security and correctness

Build: <fcntl.h> in tui.c, format-truncation in http_srv.c and /web.

WebUI:
- refuse cross-site requests (Origin must be the page itself) in the
  default no-token mode: any page could POST approval=yolo and a prompt
- session/workspace/undo/compact/attach/model changes are 409 while a
  turn runs (use-after-free, messages landing in the wrong session)
- ask_user returned "dismissed" at once instead of waiting
- cancel and shutdown wake a turn parked on a permission or question;
  SIGTERM mid-turn no longer hangs or crashes
- a page opened while a question is pending still gets it;
  Last-Event-ID resumes the stream; event seq matches the SSE id
- token mode: page assets load without the token, images via cookie
- --web-open works, --web-port/--web-token are validated
- frontend: gate keys ignored while typing, clear-images button,
  reasoning no longer wiped while streaming

HTTP server: auth and size cap before reading the body, strict
Content-Length, NUL in url_decode, 413 on keep-alive, idle timeout,
absolute-form only without a leading '/', case-insensitive Connection,
header deadline, 400 for long request lines, unlink before close,
poll() instead of select(), SO_EXCLUSIVEADDRUSE on Windows.

Markdown sanitizer escapes quotes in attribute values.

/web: passes this session's provider/model/base/effort/approval (key
via the environment), keeps output off the TUI, shows the real URL and
keeps CURRENT on the TUI's session.

Tests: scripts/e2e_webui_ask.py, scripts/e2e_webui_http.py (run from
e2e_webui.sh), sanitizer quote test.

include/cst_http.h

@@ -32,6 +32,7 @@ typedef struct {
   const char *host;
   const char *content_type;
   const char *user_agent;
+  const char *last_event_id; /* an EventSource reconnecting */
   int keep_alive;
 } cst_http_req_t;
 
@@ -72,8 +73,10 @@ cst_http_server_t *cst_http_start(const cst_http_opts_t *o, char *err,
 int cst_http_port(const cst_http_server_t *s);
 /* Non-zero once stop() was called: long-lived handlers must return. */
 int cst_http_stopping(const cst_http_server_t *s);
-/* Close the listener and every connection, then wait for the threads. */
-void cst_http_stop(cst_http_server_t *s);
+/* Close the listener and every connection, then wait (30 s at most) for the
+ * handler threads. Returns how many still run: their ud must stay alive. */
+int cst_http_stop(cst_http_server_t *s);
+/* Does nothing while handlers still run on s. */
 void cst_http_free(cst_http_server_t *s);
 
 /* --- inside a handler --- */

include/cst_version.h

@@ -3,7 +3,7 @@
 
 #define CST_VERSION_MAJOR 0
 #define CST_VERSION_MINOR 8
-#define CST_VERSION_PATCH 0
+#define CST_VERSION_PATCH 2
 
 #define CST_VERSION_STR_(x) #x
 #define CST_VERSION_STR(x) CST_VERSION_STR_(x)