feat: enhance language selection and boundary falloff features

- Added a language dropdown selector to replace the previous button-based language toggle.
- Integrated boundary falloff settings into the preview material, allowing for smoother transitions between masked and unmasked areas.
- Updated shaders to utilize boundary falloff attributes for improved visual fidelity in bump-only previews.
- Refactored related CSS styles for the new dropdown and adjusted layout for better usability.
- Introduced new functions for computing boundary attributes and managing edge data textures.

index.html

@@ -15,17 +15,6 @@
                             : window.matchMedia('(prefers-color-scheme: dark)').matches;
       if (!prefersDark) document.documentElement.setAttribute('data-theme', 'light');
     })();
-    // Apply saved language before first paint to avoid flash
-    (function() {
-      const l = localStorage.getItem('stlt-lang');
-      if (l === 'de' || l === 'en') {
-        document.documentElement.setAttribute('data-lang', l);
-        document.documentElement.setAttribute('lang', l);
-      } else if (navigator.language && navigator.language.toLowerCase().startsWith('de')) {
-        document.documentElement.setAttribute('data-lang', 'de');
-        document.documentElement.setAttribute('lang', 'de');
-      }
-    })();
   </script>
   <link rel="preconnect" href="https://cdn.jsdelivr.net" crossorigin>
   <script type="importmap">
@@ -46,8 +35,6 @@
     </div>
     <div class="header-actions">
       <div class="lang-seg">
-        <button class="lang-btn active" data-lang-code="en">EN</button>
-        <button class="lang-btn" data-lang-code="de">DE</button>
       </div>
       <button id="theme-toggle" class="theme-toggle"
         data-i18n-title="theme.toggleTitle"
@@ -237,9 +224,19 @@
         </div>
       </section>
 
-      <!-- Mask Angles -->
+      <!-- Masking -->
       <section class="panel-section">
-        <h2 data-i18n="sections.maskAngles" data-i18n-title="tooltips.maskAngles" title="0° = no masking. Surfaces within this angle of horizontal will not be textured.">Mask Angles ⓘ</h2>
+        <h2 data-i18n="sections.masking">Masking</h2>
+
+        <!-- Boundary Falloff (applies to both angle and surface masking) -->
+        <div class="form-row slider-row">
+          <label for="boundary-falloff" data-i18n="labels.boundaryFalloff" data-i18n-title="tooltips.boundaryFalloff" title="Gradually reduces displacement to zero near masked boundaries, preventing triangle overlap where textured and non-textured regions meet.">Smooth Mask ⓘ</label>
+          <input type="range" id="boundary-falloff" min="0" max="10" step="0.1" value="0" />
+          <input type="number" class="val" id="boundary-falloff-val" value="0" min="0" max="10" step="0.1" />
+        </div>
+
+        <!-- By Angle -->
+        <h3 class="panel-subsection-heading" data-i18n="sections.maskAngles" data-i18n-title="tooltips.maskAngles" title="0° = no masking. Surfaces within this angle of horizontal will not be textured.">By Angle ⓘ</h3>
         <div class="form-row slider-row">
           <label for="bottom-angle-limit" data-i18n="labels.bottomFaces" data-i18n-title="tooltips.bottomFaces" title="Suppress texture on downward-facing surfaces within this angle of horizontal">Bottom faces</label>
           <input type="range" id="bottom-angle-limit" min="0" max="90" step="1" value="5" />
@@ -250,11 +247,9 @@
           <input type="range" id="top-angle-limit" min="0" max="90" step="1" value="0" />
           <input type="number" class="val" id="top-angle-limit-val" value="0" min="0" max="90" step="1" />
         </div>
-      </section>
 
-      <!-- Surface Exclusions -->
-      <section class="panel-section">
-        <h2 id="excl-section-heading" data-i18n="sections.surfaceMasking" data-i18n-title="tooltips.surfaceMasking" title="Mask surfaces to control which areas receive displacement.">Surface Masking ⓘ</h2>
+        <!-- By Surface -->
+        <h3 class="panel-subsection-heading" id="excl-section-heading" data-i18n="sections.surfaceMasking" data-i18n-title="tooltips.surfaceMasking" title="Mask surfaces to control which areas receive displacement.">By Surface ⓘ</h3>
         <p id="excl-hint" class="excl-hint" style="display:none"></p>
 
         <!-- Mode toggle: Exclude / Include Only -->