Let conversation view off mean off

The setting reached only as far as the query. It set `collapseThreads`, so the
list correctly showed individual messages -- and then everything downstream
carried on working in threads. Opening one message highlighted every row of its
thread and filled the reading pane with the whole conversation, which is the
grouping the setting was turned off to avoid. The empty pane went on offering
"62 conversations" either way.

Three places had to learn about it, and the two rules behind them now live
together in lib/openMessage.ts:

- the row highlight matched on threadId, so siblings lit up
- ThreadView rendered every message the thread held
- the empty state named conversations regardless

The thread id stays in the path and loading is unchanged; the opened message
rides in `m`. Keeping it in the URL rather than in memory is what makes a
reload or a shared link come back to the same message, and an id that names
nothing in the thread falls back to the conversation -- which is what a link
from somebody with the setting on looks like, and what a stale parameter looks
like after switching back. Better a conversation than an empty pane.

Nine catalogues gain "No message selected" and "Select a message to read it
here"; "{n} messages" was already there, plural forms and all.

web/src/views/mail/ThreadView.tsx

@@ -1,6 +1,7 @@
 import { useCallback, useEffect, useMemo, useRef, useState } from "react";
 import { AlertOctagon, Archive, ArrowLeft, ChevronDown, ChevronUp, FolderInput, Forward, Mail, MailOpen, MailPlus, MoreVertical, Printer, Reply, ReplyAll, ShieldCheck, Star, Tag, Trash2, Download , Paperclip} from "lucide-react";
 import { useMail } from "@/store/mail";
+import { visibleMessages } from "@/lib/openMessage";
 import { useSettings } from "@/store/settings";
 import { useCompose } from "@/store/compose";
 import type { Email, Id } from "@/jmap/types";
@@ -25,9 +26,15 @@ interface Props {
   onNavigate: (delta: number) => void;
   hasPrev: boolean;
   hasNext: boolean;
+  /**
+   * With conversation view off, the single message to show. The thread is still
+   * what loads -- one request, and the reply/forward paths keep the context
+   * they need -- but only this message is rendered.
+   */
+  messageId?: Id | null;
 }
 
-export function ThreadView({ threadId, mailboxId, onBack, actions, onNavigate, hasPrev, hasNext }: Props) {
+export function ThreadView({ threadId, mailboxId, onBack, actions, onNavigate, hasPrev, hasNext, messageId = null }: Props) {
   const loadThread = useMail((s) => s.loadThread);
   const thread = useMail((s) => s.threads[threadId]);
   const emails = useMail((s) => s.emails);
@@ -82,8 +89,9 @@ export function ThreadView({ threadId, mailboxId, onBack, actions, onNavigate, h
       if (junk && e.mailboxIds[junk]) return false;
       return true;
     });
-    return (filtered.length ? filtered : all).sort((a, b) => a.receivedAt.localeCompare(b.receivedAt));
-  }, [thread, emails, fullIds, mailboxId]);
+    const shown = filtered.length ? filtered : all;
+    return visibleMessages(shown, messageId).sort((a, b) => a.receivedAt.localeCompare(b.receivedAt));
+  }, [thread, emails, fullIds, mailboxId, messageId]);
 
   /*
    * Which messages were unread when this conversation was opened.