feat: say which way the unread action will go, and hide it when it cannot

Item 112, and 99 and 147 with it: the user's note is one design across
all three. A union is not a state. ThreadSummary::tags is notmuch's
union over the conversation, so a thread holding even one unread message
answered "unread" and the thread toggle always chose "mark read". There
was no input that reached "mark thread unread" on a mixed thread, which
is the thread a user wants it for.

The thread toggle becomes two absolute actions, mark_thread_read and
mark_thread_unread. Neither takes a default chord, at the user's choice:
Ctrl+Alt+U meant whichever direction the union picked, and since item
132 a shortcut is a chosen subset rather than a requirement. It is now
unbound.

The message-scoped toggle stays a toggle, because one message has a real
two-valued state, and its label now names the direction it will go. On a
selection with no single state the entry is hidden rather than labelled
wrongly, chosen over disabling it; the thread submenu is the route then,
and its entries are absolute.

selectionTagPresence() is the three-valued predicate that needed to
exist. everySelectedRowHasTag() delegates to it and keeps its two-valued
answer, which is all a direction needs; a label needs the third value.
The refresh is keyed on the model's dataChanged as well as on the
selection, so a write moves the label without reselecting and none of
the six optimistic-update call sites has to remember.

Three mutations fail: restoring the union predicate reports the user's
original symptom, showing the action on a mixed selection, and dropping
the dataChanged refresh. The suite is 37 of 38, the failure being item
136 on an unrelated path. Four new strings translated, lrelease reports
0 unfinished.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HFuRPtzFrSxCQjFk6tq7gD

1 files changed, 32 insertions, 4 deletions

diff --git a/translations/qtmaildir_it_IT.ts b/translations/qtmaildir_it_IT.ts
index 913d151..a976f4e 100644
--- a/translations/qtmaildir_it_IT.ts
+++ b/translations/qtmaildir_it_IT.ts
@@ -499,6 +499,14 @@ Il messaggio È stato inviato. Non inviarlo di nuovo.</translation>
         <translation>Aggiunge o rimuove l&apos;etichetta deleted</translation>
     </message>
     <message>
+        <source>Mark thread &amp;read</source>
+        <translation>Segna conversazione come &amp;letta</translation>
+    </message>
+    <message>
+        <source>Remove the unread tag from every message of the selected threads</source>
+        <translation>Rimuove il tag unread da ogni messaggio delle conversazioni selezionate</translation>
+    </message>
+    <message>
         <source>Re&amp;ply</source>
         <translation>Ris&amp;pondi</translation>
     </message>
@@ -752,14 +760,18 @@ Il messaggio È stato inviato. Non inviarlo di nuovo.</translation>
         <translation>Segna conversazione come spam</translation>
     </message>
     <message>
-        <source>Toggle the unread tag on whole threads</source>
-        <translation>Inverte l&apos;etichetta non letto su intere conversazioni</translation>
-    </message>
-    <message>
         <source>Mark thread read</source>
         <translation>Segna conversazione come letta</translation>
     </message>
     <message>
+        <source>Mark thread &amp;unread</source>
+        <translation>Segna conversazione come &amp;non letta</translation>
+    </message>
+    <message>
+        <source>Add the unread tag to every message of the selected threads</source>
+        <translation>Aggiunge il tag unread a ogni messaggio delle conversazioni selezionate</translation>
+    </message>
+    <message>
         <source>Mark thread unread</source>
         <translation>Segna conversazione come non letta</translation>
     </message>
@@ -1164,6 +1176,22 @@ Il messaggio È stato inviato. Non inviarlo di nuovo.</translation>
             <numerusform>%1: %n conversazioni</numerusform>
         </translation>
     </message>
+    <message>
+        <source>Mark as &amp;read</source>
+        <translation>Segna come &amp;letto</translation>
+    </message>
+    <message>
+        <source>Remove the unread tag from the selection</source>
+        <translation>Rimuove il tag unread dalla selezione</translation>
+    </message>
+    <message>
+        <source>Mark as &amp;unread</source>
+        <translation>Segna come &amp;non letto</translation>
+    </message>
+    <message>
+        <source>Add the unread tag to the selection</source>
+        <translation>Aggiunge il tag unread alla selezione</translation>
+    </message>
     <message numerus="yes">
         <source>1 thread selected (%n message(s))</source>
         <translation>