a11y: scope=col + aria-labelledby on the template-guide variable table

The "Verfugbare Variablen / Available Variables" reference table inside the Template Guide modal (a 3-column data table mapping placeholders to descriptions and example output) was missing two standard data-table a11y hooks:

- The three <th> cells had no scope attribute. The implicit <th>-inside-<thead> column scope works in modern screen readers but explicit scope="col" is the recommended pattern and what a11y audit tooling looks for.

- The <table> had no programmatic relationship to its own heading (the <h3 id="templateGuideVarsTitle"> directly above). Added aria-labelledby="templateGuideVarsTitle" so screen readers announce the table's purpose ("Verfugbare Variablen, Tabelle") when the user enters it.

Pairs nicely with the storage-stats table fix in 4.6.107 — both data tables in the app now have explicit column scopes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/index.html

@@ -160,12 +160,12 @@
 
             <h3 id="templateGuideVarsTitle" class="template-guide-vars-title">Verfugbare Variablen</h3>
             <div class="template-guide-table-wrap">
-                <table class="template-guide-table">
+                <table class="template-guide-table" aria-labelledby="templateGuideVarsTitle">
                     <thead>
                         <tr>
-                            <th id="templateGuideVarCol">Variable</th>
+                            <th id="templateGuideVarCol" scope="col">Variable</th>
-                            <th id="templateGuideDescCol">Beschreibung</th>
+                            <th id="templateGuideDescCol" scope="col">Beschreibung</th>
-                            <th id="templateGuideExampleCol">Beispiel</th>
+                            <th id="templateGuideExampleCol" scope="col">Beispiel</th>
                         </tr>
                     </thead>
                     <tbody id="templateGuideBody"></tbody>