ui: rename tone "Novice" → "Pro"; fit tone-toggle to longest option

User-visible relabel only. Backend tone value stays NOVICE — no API
contract change, no migration on stored user.digest_tone, the
glossary/plain-prose depth of analysis is unchanged. The marketing
intent is that "Pro" reads better than "Novice" on the dashboard
header; landing/pricing/privacy copy still uses the word "Novice" in
flowing prose, so leaving those alone keeps the existing explanations
coherent until they get a copy pass.

Toggle width: the popup expansion (positioned left:0/right:0) is
sized by the container, which previously sized to the active button.
When "Pro" was active the popup was too narrow to fit "Intermediate".
Bumped .tone-toggle button min-width to 10em so both buttons reserve
enough room for the longest label regardless of which one is active.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/templates/base.html

@@ -234,10 +234,14 @@
         <a href="/log"  class="{% if request.url.path.startswith('/log') %}active{% endif %}">Log</a>
       </nav>
       <div class="header-right">
+        {# The "Pro" label maps to the NOVICE tone server-side — kept that
+           way to avoid touching every stored user preference and API
+           contract. The mode itself (glossary tooltips + plainer
+           framing) is unchanged; only the display label changes. #}
         <div id="tone-toggle" class="tone-toggle" data-tone="INTERMEDIATE"
              role="group" aria-label="Explanation level">
           <button type="button" data-value="NOVICE"
-                  onclick="cassandraSetTone('NOVICE')">Novice</button>
+                  onclick="cassandraSetTone('NOVICE')">Pro</button>
           <button type="button" data-value="INTERMEDIATE"
                   onclick="cassandraSetTone('INTERMEDIATE')">Intermediate</button>
         </div>