fix(diagnostics): deep-review round 1 — a send that cannot race, lose or repeat itself

The send, the draft and the screenshots take the session's row lock, so an
autosave or a paste during a send waits for it and then sees it, and two sends
cannot open two issues. A save keeps the delivery's progress; a failed edit
that shows the screenshots is finished by the retry; an answer that is not the
tracker's API (a redirect, a proxy page) is refused, not a crash that loses the
issue's record. Only an attempt that opened an issue counts toward the pace.

Screenshots: deleting one after the send is refused, a user's storage is
capped across sessions, new sessions and zip exports are paced, images are
stored uncompressed in the zip. One liveness rule, one session view, one image
type table.

Logging: a session's elevation is tracked apart from the user's own, so the
browser's own requests (no header) no longer lower the floor a session needs;
Stop ends it at once.

Composer: an image pasted anywhere is taken; what is still unsaved is saved on
close, and "saved" is shown only when it is; the draft is held while it is
sent; a failed screenshot read says so instead of reading as none; focus moves
to what stays; the preview follows the text size. Pill: the step prompt and a
running Stop survive the pill moving between hosts, the prompt keeps the
dialog's Tab trap, the toast region lets clicks through, other tabs hear of a
Start or Stop, the last seconds are sent before the cap, a capture with no
frame is an error.

frontend/src/lib/debugRecorder.ts

@@ -10,6 +10,34 @@ export const openRecorderStart = (): void => startOpen.set(true);
 export const closeRecorderStart = (): void => startOpen.set(false);
 export const useRecorderStartOpen = (): boolean => startOpen.use();
 
+// The pill's step prompt — open, and what is typed in it — held here, not in the pill: the pill
+// rides the topmost toast host, and a dialog or the player opening or closing moves it to another
+// one (a remount). The prompt must come along with what was typed.
+const stepPrompt = createStore<string | null>(null);
+/** The step prompt's text while it is open, else null. */
+export const useStepPrompt = (): string | null => stepPrompt.use();
+export const setStepPrompt = (text: string | null): void => stepPrompt.set(text);
+
+// Other tabs of this browser: a Start or a Stop in one is news to the rest, which otherwise learn of
+// it only at their next `me` poll — and meanwhile record into a session that has ended.
+const CHANNEL = "siftlode.debugSession";
+let channel: BroadcastChannel | null | undefined;
+function tabs(): BroadcastChannel | null {
+  if (channel === undefined)
+    channel = typeof BroadcastChannel === "undefined" ? null : new BroadcastChannel(CHANNEL);
+  return channel;
+}
+/** Tell the other tabs the recording started or stopped. */
+export const announceSessionChange = (): void => tabs()?.postMessage("changed");
+/** Run `fn` when another tab started or stopped a recording. */
+export function onSessionChangeElsewhere(fn: () => void): () => void {
+  const c = tabs();
+  if (!c) return () => {};
+  const handler = () => fn();
+  c.addEventListener("message", handler);
+  return () => c.removeEventListener("message", handler);
+}
+
 const composing = createStore<string | null>(null);
 /** Open the issue composer for session `id` — after Stop, when the cap ends it, from Settings. */
 export const openComposer = (id: string): void => composing.set(id);