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/de.ts

@@ -756,6 +756,7 @@ export const catalog: Catalog = {
     "Open the Mail view to see all shortcuts.": "Öffnen Sie die E-Mail-Ansicht, um alle Tastenkürzel zu sehen.",
     "Gmail-style shortcuts are always on. Press {key} anywhere to see this list.": "Tastenkürzel im Gmail-Stil sind immer aktiv. Drücken Sie überall {key}, um diese Liste zu sehen.",
     "Select a conversation to read it here · Press {key} for shortcuts": "Wählen Sie eine Konversation, um sie hier zu lesen · {key} für Tastenkürzel",
+    "Select a message to read it here · Press {key} for shortcuts": "Wählen Sie eine Nachricht, um sie hier zu lesen · {key} für Tastenkürzel",
     "Tip: press {key} on a conversation to apply labels. Search with {operator}.": "Tipp: Drücken Sie {key} auf einer Konversation, um Labels zu vergeben. Suchen Sie mit {operator}.",
     "A fast, friendly, open-source webmail for {server}, built on JMAP.": "Eine schnelle, freundliche Open-Source-Webmail für {server}, auf JMAP aufgebaut.",
     "Defaults for the calendar views and new events.": "Vorgaben für die Kalenderansichten und neue Termine.",
@@ -834,6 +835,7 @@ export const catalog: Catalog = {
     "Nothing": "Nichts",
 
     "No conversation selected": "Keine Konversation ausgewählt",
+    "No message selected": "Keine Nachricht ausgewählt",
     "Drop here for the top level": "Hierher ziehen für die oberste Ebene",
 
     // ── Remaining prose ────────────────────────────────────────────────