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/AppCommandPalette.tsx

@@ -36,7 +36,8 @@ export default function AppCommandPalette() {
   const { setSelectedId } = usePlaylistsActions();
   const { theme, setTheme } = usePrefs();
   const savedViews = useSavedViews();
-  const recording = useDebugRecording();
+  const { session: recordingSession, stop: stopRecording, markStep } = useDebugRecording();
+  const stopMutate = stopRecording.mutate;
 
   // Only fetch the lists while the palette is actually open — it is a rarely-opened surface and
   // these are otherwise the feed's and the playlist rail's own queries, shared through the cache.
@@ -155,19 +156,19 @@ export default function AppCommandPalette() {
         // player or a dialog where the pill's buttons are a long Tab away.
         ...(me?.is_demo
           ? []
-          : recording.session
+          : recordingSession
             ? [
                 {
                   id: "action:recorder:step",
                   label: t("recorder.palette.step"),
                   icon: <Flag className="w-4 h-4" />,
-                  run: () => recording.markStep(""),
+                  run: () => markStep(""),
                 },
                 {
                   id: "action:recorder:stop",
                   label: t("recorder.palette.stop"),
                   icon: <Square className="w-4 h-4" />,
-                  run: () => recording.stop.mutate(recording.session!.id),
+                  run: () => stopMutate(recordingSession.id),
                 },
               ]
             : [
@@ -197,7 +198,9 @@ export default function AppCommandPalette() {
     savedViews,
     theme,
     setTheme,
-    recording,
+    recordingSession,
+    markStep,
+    stopMutate,
     setPage,
     openChannel,
     setSelectedId,