feat(brief): besluit guidance + read-only notice (role clarity)

Surface the besluit-driven assistance that was previously silent: a pure
besluitGuidance() (kern passage count + needs-reason flag) rendered as a hint below
the besluit panel in behandel-scherm (warning to pick a reden, else info on how many
standaardteksten were inserted). Add a read-only notice on letter-composer for a pure
viewer (no edit/approve/reject/send right, e.g. admin) so the read-only letter isn't
mistaken for a broken editor — the "reverted to a view of the letter" confusion was a
role issue (non-drafter sees the composer). +besluitGuidance spec.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/locale/messages.en.xlf

@@ -290,6 +290,18 @@
           <context context-type="linenumber">158</context>
         </context-group>
       </trans-unit>
+      <trans-unit id="brief.guidance.needsReason" datatype="html">
+        <source>Kies een reden, zodat de juiste motivering aan de brief wordt toegevoegd.</source>
+        <target datatype="html">Choose a reason so the correct justification is added to the letter.</target>
+      </trans-unit>
+      <trans-unit id="brief.guidance.inserted" datatype="html">
+        <source><x id="count" equiv-text="n"/> standaardtekst(en) toegevoegd op basis van het besluit. Vul aan met vrije tekst waar nodig.</source>
+        <target datatype="html"><x id="count" equiv-text="n"/> standard passage(s) added based on the decision. Add free text where needed.</target>
+      </trans-unit>
+      <trans-unit id="brief.readonlyNotice" datatype="html">
+        <source>Alleen-lezen weergave. De behandelaar stelt de brief op.</source>
+        <target datatype="html">Read-only view. The case handler composes the letter.</target>
+      </trans-unit>
       <trans-unit id="brief.submit" datatype="html">
         <source>Indienen ter beoordeling</source>
         <target datatype="html">Submit for review</target>