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.

style.css

@@ -47,31 +47,31 @@
   flex-shrink: 0;
 }
 
-.lang-btn {
+.lang-dropdown {
   height: 28px;
-  padding: 0 10px;
+  padding: 0 24px 0 10px; /* Add right padding for default select arrow if present */
   background: var(--surface2);
   border: none;
-  border-radius: 0;
   color: var(--text-muted);
   font-size: 11px;
   font-weight: 600;
   letter-spacing: 0.05em;
   cursor: pointer;
+  appearance: none; /* Attempt to remove default styling */
+  -moz-appearance: none;
+  -webkit-appearance: none;
+  background-image: url("data:image/svg+xml;charset=UTF-8,%3Csvg xmlns='http://www.w3.org/2000/svg' width='8' height='5' viewBox='0 0 8 5'%3E%3Cpath fill='%2366667a' d='M0 0l4 4 4-4H0z'/%3E%3C/svg%3E"); /* Custom arrow */
+  background-repeat: no-repeat;
+  background-position: right 8px center;
   transition: background 0.15s, color 0.15s;
 }
 
-.lang-btn:not(:last-child) {
-  border-right: 1px solid var(--border);
-}
-
-.lang-btn:hover {
+.lang-dropdown:hover {
   color: var(--accent);
 }
 
-.lang-btn.active {
-  background: var(--accent);
-  color: #fff;
+.lang-dropdown:focus {
+  outline: none;
 }
 
 /* ── Theme toggle button ─────────────────────────────────────────────── */
@@ -391,6 +391,15 @@ main {
   margin-bottom: 10px;
 }
 
+.panel-subsection-heading {
+  font-size: 9px;
+  font-weight: 600;
+  text-transform: uppercase;
+  letter-spacing: 0.06em;
+  color: var(--text-muted);
+  margin: 14px 0 8px;
+}
+
 /* ── Preset grid ─────────────────────────────────────────────────────── */
 .preset-grid {
   display: grid;