Give the mail list the gestures a phone already has

ihasmail's mail list was built for a mouse. A row is clicked, right-clicked
and dragged into a folder, and on a touchscreen two of those three do not
exist -- so the phone layout had the shape of a mail app and none of the
handling, and the things people reach for first simply did nothing.

Four gestures, all touch-only, so a mouse keeps drag-to-folder unchanged:

- Swipe a row sideways to act on it. Each direction is a setting -- right
  archives and left deletes by default, matching the app the phone came
  with -- and the strip revealed behind the row names what will happen in
  the folder it is happening in: "Delete forever" out of Deleted Items,
  "Not spam" inside Junk Mail, and nothing at all where the action is a
  no-op, in which case the row will not move that way.
- Hold a row to select it. Selection was reachable already, by aiming at a
  checkbox beside an avatar, which is not how anyone selects mail on a
  phone. The selection toolbar gained an overflow menu at the same time:
  report spam, mark unread and label were hidden on narrow screens and had
  nowhere else to be, so touch selection could not reach them at all.
- Hold a folder for the menu its ⋮ button opens.
- Pull the list down to refresh, and drag in from the left edge of a
  conversation to go back. The toolbar's button and arrow both stay: a
  gesture with no visible control is one only the people who already know
  about it can use.

The arithmetic behind them is in lib/touch.ts, away from the components and
under test, because the numbers are the whole thing: an axis lock biased
towards the vertical, so a diagonal flick stays a scroll rather than
deleting whatever it passes over.

Two layout bugs turned up while checking this on a 390px screen, both
older than the gestures. The app shell is a grid with only its rows named,
so it took an implicit auto column sized to the top bar's min-content --
about 470px -- and every message row ran off the right of the glass with
its date beyond the edge. The column is now stated as minmax(0, 1fr), and
the search field is allowed to shrink. Full-screen surfaces measure in dvh
rather than vh, and the tab bar, drawer and compose button keep out from
under the notch and the home indicator.

web/src/views/mail/ThreadView.tsx

@@ -7,10 +7,11 @@ import type { Email, Id } from "@/jmap/types";
 import { MessageView } from "./MessageView";
 import type { ListActions } from "./MessageList";
 import { MenuItem, MenuSep, Popover, useMenu } from "@/ui/popover";
-import { Spinner } from "@/ui/misc";
+import { Spinner, useIsNarrow, useIsTouch } from "@/ui/misc";
 import { client } from "@/jmap/client";
 import { LabelPicker } from "./LabelPicker";
 import { threadScrollTarget } from "@/lib/threadScroll";
+import { useEdgeBack } from "@/lib/touch";
 
 /** How long the opening scroll keeps its place while bodies and images land. */
 const HOLD_MS = 2000;
@@ -42,6 +43,18 @@ export function ThreadView({ threadId, mailboxId, onBack, actions, onNavigate, h
   const moreMenu = useMenu();
   const scrollRef = useRef<HTMLDivElement>(null);
   const markTimer = useRef<number | null>(null);
+  const isTouch = useIsTouch();
+  const narrow = useIsNarrow();
+  /*
+   * Drag in from the left edge to go back to the list.
+   *
+   * Only where back means something: on a wide screen the list is still
+   * beside the conversation and there is nowhere to go. The toolbar's arrow
+   * stays regardless — a gesture with no visible control is a gesture only
+   * the people who already know about it can use.
+   */
+  const [viewEl, setViewEl] = useState<HTMLDivElement | null>(null);
+  useEdgeBack(viewEl, onBack, isTouch && narrow);
 
   // Load
   useEffect(() => {
@@ -215,7 +228,7 @@ export function ThreadView({ threadId, mailboxId, onBack, actions, onNavigate, h
   const accountId = useMail((s) => s.accountId);
 
   return (
-    <div className="thread-view">
+    <div className="thread-view" ref={setViewEl}>
       <div className="thread-toolbar">
         <button className="icon-btn" onClick={onBack} aria-label="Back to list" title="Back (u)">
           <ArrowLeft size={20} />