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/es.ts

@@ -811,6 +811,7 @@ export const catalog: Catalog = {
     "Rename folder": "Cambiar el nombre de la carpeta",
     "Search: {query}": "Búsqueda: {query}",
     "No conversation selected": "Ninguna conversación seleccionada",
+    "No message selected": "Ningún mensaje seleccionado",
     "Drop here for the top level": "Suelte aquí para el nivel superior",
 
     // ── Longer prose ───────────────────────────────────────────────────
@@ -819,6 +820,7 @@ export const catalog: Catalog = {
     "Open the Mail view to see all shortcuts.": "Abra la vista de Correo para ver todos los atajos.",
     "Gmail-style shortcuts are always on. Press {key} anywhere to see this list.": "Los atajos al estilo de Gmail están siempre activos. Pulse {key} en cualquier momento para ver esta lista.",
     "Select a conversation to read it here · Press {key} for shortcuts": "Seleccione una conversación para leerla aquí · {key} para los atajos",
+    "Select a message to read it here · Press {key} for shortcuts": "Seleccione un mensaje para leerlo aquí · {key} para los atajos",
     "Tip: press {key} on a conversation to apply labels. Search with {operator}.": "Consejo: pulse {key} sobre una conversación para aplicar etiquetas. Busque con {operator}.",
     "A fast, friendly, open-source webmail for {server}, built on JMAP.": "Un webmail libre, rápido y agradable para {server}, construido sobre JMAP.",
     "Defaults for the calendar views and new events.": "Valores predeterminados de las vistas del calendario y de los eventos nuevos.",