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/zh-Hans.ts

@@ -746,6 +746,7 @@ export const catalog: Catalog = {
     "Open the Mail view to see all shortcuts.": "打开邮件视图以查看全部快捷键。",
     "Gmail-style shortcuts are always on. Press {key} anywhere to see this list.": "Gmail 风格的快捷键始终启用。在任意位置按 {key} 即可查看此列表。",
     "Select a conversation to read it here · Press {key} for shortcuts": "选择一个会话即可在此阅读 · 按 {key} 查看快捷键",
+    "Select a message to read it here · Press {key} for shortcuts": "选择一封邮件即可在此阅读 · 按 {key} 查看快捷键",
     "Tip: press {key} on a conversation to apply labels. Search with {operator}.": "提示：在会话上按 {key} 可添加标签。使用 {operator} 搜索。",
     "A fast, friendly, open-source webmail for {server}, built on JMAP.": "一款面向 {server} 的快速、友好的开源网页邮箱，基于 JMAP 构建。",
 
@@ -842,6 +843,7 @@ export const catalog: Catalog = {
     "Not spam": "不是垃圾邮件",
     "Nothing": "不执行任何操作",
     "No conversation selected": "未选择会话",
+    "No message selected": "未选择邮件",
     "Drop here for the top level": "拖放到此处可移至顶层",
     "Later today": "今天晚些时候",
     "Tomorrow morning": "明天上午",