fix(diagnostics): deep-review round 3 — a keyboard owner that is not a Tab trap

Round 2 made the step prompt a blocking layer so that `j` and the `g` chords
would not reach the page behind it. The dialog it sits in gave up its Tab trap
because of that, while the prompt's own replacement trap covered only its own
two controls — so Tab from anywhere else in the dialog (the bar's Screenshot or
Stop, the dialog's own buttons) walked out behind the scrim. `layerStack` now
separates the two questions it had folded into one flag: a layer can own the
keyboard and still leave the Tab trap to the dialog below it. The mirror case,
a dialog opening over the prompt, falls out of the same split.

A send's claim is now a lease with an owner. Every progress report re-stamps
it, so a send slower than the lease cannot lose it while it is still running,
and only the send that took a claim can give it back. The issue_create audit
row is staged inside the send and rides the same checkpoint as `issue_url`, so
the trail cannot be missing for an issue that exists. The attachment-name
prefix has one definition, shared by the producer and both readers.

A running send is reported to the UI. A reload or a second tab used to render
it exactly like a send that STOPPED, and offered a retry the server was certain
to refuse; the composer now says the report is being sent, holds the draft, and
puts both the retry and Create issue out. The past-sessions list says the same
for the same reason — it is the list a user opens to check on a send, and both
"draft" and "half sent" describe one that died. `/preview` and the retry path
got a pace of their own, and the per-user byte cap took the user-scoped lock
that makes it a cap instead of a guess.

Verified in a real browser on localdev: Tab cycles the whole dialog with the
prompt open, the prompt still swallows the page's keys, and the issue link is
focusable where the composer puts it. All 24 regression tests mutation-checked.

frontend/src/lib/api/core.ts

@@ -360,9 +360,11 @@ export async function req<T = unknown>(
       reportFailure(err, !!cfg.quiet);
       throw err;
     }
-    // A file (`cfg.raw`): the answer itself, for the caller to read as a blob. A download that went
-    // out as a plain `<a href>` instead would carry no refusal the app could show — the browser
-    // would simply save the error.
+    // A file (`cfg.raw`): the answer itself, for the caller to read as a blob. For a SMALL generated
+    // file whose endpoint can refuse (a report, an export): sent as a plain `<a href>` it would
+    // carry no refusal the app could show — the browser would simply save the error page. A large
+    // stored file stays a plain navigation (the download centre, the Plex original), so the browser
+    // streams it to disk instead of buffering gigabytes into the tab.
     if (cfg.raw) return r as T;
     // The single audited cast in this layer: the server contract is asserted here, once, so
     // every typed api.* method (T comes from its declared return type) is checked from here up.