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/components/ui/Toaster.dom.test.tsx

@@ -50,6 +50,14 @@ describe("Toaster — one host shows the toasts", () => {
     app.unmount();
   });
 
+  it("lets a click through wherever there is no toast and nothing docked, in every host", () => {
+    render(<Toaster />);
+    const region = document.querySelector<HTMLElement>("[data-toast-region]")!;
+    // The empty part of the region (beside and above a docked control) must not swallow the page's
+    // clicks; the cards and the dock opt back in.
+    expect(region.className).toContain("pointer-events-none");
+  });
+
   it("does not let a click on a toast reach the overlay's backdrop", () => {
     const backdrop = vi.fn();
     render(