Fix nav mode strip: underline-tab style + stable scrollbar gutter

Replace button-style strip with proper underline tabs — active side gets
an ember bottom-border matching the sidebar's existing active-link
language. Add scrollbar-gutter: stable so the sidebar interior width
does not shift when the scrollbar appears/disappears between modes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/Views/Shared/_Layout.cshtml

@@ -180,6 +180,7 @@
             z-index: 1000;
             transition: all 0.3s ease;
             overflow-y: auto;
+            scrollbar-gutter: stable;
             display: flex;
             flex-direction: column;
         }
@@ -334,28 +335,31 @@
 
         /* ── Nav mode strip (Operations | Finance tab switcher) ──────── */
         .nav-mode-strip {
-            display: grid;
-            grid-template-columns: 1fr 1fr;
-            gap: 3px;
-            padding: 8px 12px 10px;
-            border-bottom: 1px solid rgba(255,255,255,0.08);
+            display: flex;
+            border-bottom: 1px solid rgba(255,255,255,0.1);
+            padding: 0 8px;
+            margin-bottom: 4px;
+            flex-shrink: 0;
         }
 
         .nav-mode-btn {
+            flex: 1;
             display: flex;
             align-items: center;
             justify-content: center;
             gap: 5px;
-            padding: 7px 6px;
+            padding: 10px 4px 8px;
             border: none;
-            border-radius: 5px;
+            border-bottom: 2px solid transparent;
+            border-radius: 0;
+            margin-bottom: -1px;
             font-family: var(--font-mono, 'IBM Plex Mono', ui-monospace, monospace);
             font-size: 0.64rem;
             font-weight: 500;
             text-transform: uppercase;
             letter-spacing: 0.08em;
             cursor: pointer;
-            transition: background 0.15s, color 0.15s;
+            transition: color 0.15s, border-color 0.15s;
             background: transparent;
             color: rgba(255,255,255,0.38);
             white-space: nowrap;
@@ -367,13 +371,12 @@
         }
 
         .nav-mode-btn:hover {
-            background: rgba(255,255,255,0.08);
-            color: rgba(255,255,255,0.8);
+            color: rgba(255,255,255,0.75);
         }
 
         .nav-mode-btn.active {
-            background: var(--pcl-ember, #d97706);
             color: #fff;
+            border-bottom-color: var(--pcl-ember, #d97706);
         }
 
         /* Main Content */