feat: add brand assets, design tokens and light/dark theme

- Use real logo/isotype SVGs across app and emails
- Token-based color system aligned with landing (brand scale + semantic tokens)
- ThemeProvider (light/dark/system) with header toggle and settings card
- Embed logo in transactional emails; centralize email colors

apps/web/public/favicon.svg

@@ -0,0 +1,12 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="256" height="256" viewBox="718.5 93 144 144">
+  <title>Gruperly</title>
+  <style>
+    .branch { fill: #171D54; }
+    .diag { fill: #93B4FD; }
+    @media (prefers-color-scheme: dark) {
+      .branch { fill: #EEF4FF; }
+    }
+  </style>
+  <path class="branch" d="M755 111 H767.4 Q771.3 111 772.6 114.8 C776.5 126.2 781.3 137.6 787.6 148.9 Q789.2 151.9 785.8 152.5 C781.9 153.2 777.8 153.4 774 152.9 Q770.2 152.4 768.5 149 C762.9 138 757.9 126.7 753.1 115.3 Q751.3 111 755 111 Z"/>
+  <path class="diag" d="M812.5 111 H824.4 Q828.8 111 827.2 115.1 C817.2 140.2 807.3 167 796.4 194.6 C790.1 210.7 783.2 219 767.2 219 C764.2 219 761.5 218.7 759 218.2 Q756.6 217.7 757 214.7 L758 205.1 Q758.3 202.4 761.1 203 C763.1 203.5 765.2 203.9 767 203.9 C774.9 203.9 778 198.7 781.6 189.4 C791.9 162.5 800.1 138.1 808.4 114 Q809.5 111 812.5 111 Z"/>
+</svg>