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/locales/ja.ts

@@ -747,6 +747,7 @@ export const catalog: Catalog = {
     "Open the Mail view to see all shortcuts.": "すべてのショートカットはメール画面で確認できます。",
     "Gmail-style shortcuts are always on. Press {key} anywhere to see this list.": "Gmail 形式のショートカットは常に有効です。どこでも {key} を押すとこの一覧を表示します。",
     "Select a conversation to read it here · Press {key} for shortcuts": "スレッドを選ぶとここに表示されます · {key} でショートカット一覧",
+    "Select a message to read it here · Press {key} for shortcuts": "メールを選ぶとここに表示されます · {key} でショートカット一覧",
     "Tip: press {key} on a conversation to apply labels. Search with {operator}.": "ヒント: スレッド上で {key} を押すとラベルを付けられます。検索には {operator} が使えます。",
     "A fast, friendly, open-source webmail for {server}, built on JMAP.": "{server} のための、軽快で使いやすいオープンソースのウェブメール。JMAP で動作します。",
 
@@ -843,6 +844,7 @@ export const catalog: Catalog = {
     "Not spam": "迷惑メールではない",
     "Nothing": "何もしない",
     "No conversation selected": "スレッドが選択されていません",
+    "No message selected": "メールが選択されていません",
     "Drop here for the top level": "ここにドロップすると最上位へ移動します",
     "Later today": "今日のうちに",
     "Tomorrow morning": "明日の朝",