feat(frontend): terminal restyle + dm + grouping + brand override

- Restyle: ink 'blueprint' surfaces + IBM Plex Mono / Major Mono
  Display, hairline borders, restrained motion, 12px radii. Fabric
  purple accent kept. Layout unchanged.
- Brand override (build-time VITE_ only): VITE_APP_NAME / VITE_LOGO_URL
  / VITE_FAVICON_URL via src/lib/brand.ts (applyBrand in main.tsx);
  login wordmark/logo + title + favicon. .env.example documented.
- dm x-type: create modal participant becomes single-select (radio)
  for dm and forces private; member right-click menu gains 'Create
  new DM channel' (non-unique, fresh channel each time).
- Channels sidebar grouped by x-type with group headers.

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

src/index.css

@@ -1,28 +1,32 @@
 :root {
-  --rail: #18191c;
-  --side: #1f2125;
-  --main: #26282d;
-  --elevated: #1a1b1e;
-  --input: #1a1b1e;
-  --hover: rgba(255, 255, 255, 0.04);
-  --active: rgba(168, 85, 247, 0.16);
+  /* Cryptic lab terminal: deep "blueprint" ink surfaces, monospace
+     everything, restrained motion. Accent stays Fabric purple. */
+  --ink: #080a0d;
+  --rail: #0b0e12;
+  --side: #0f141b;
+  --main: #0a0d12;
+  --elevated: #11161d;
+  --input: #0b0e12;
+  --hover: rgba(168, 85, 247, 0.1);
+  --active: rgba(168, 85, 247, 0.18);
 
-  --text: #dbdee1;
-  --text-muted: #949ba4;
-  --text-faint: #6d7178;
-  --text-h: #f2f3f5;
+  --text: #c7d0dc;
+  --text-muted: #6f7d92;
+  --text-faint: #4f5b6b;
+  --text-h: #e9f0fa;
 
-  --border: #303237;
+  --border: #1d2733;
   --accent: #a855f7;
   --accent-strong: #9333ea;
-  --accent-soft: rgba(168, 85, 247, 0.14);
-  --danger: #f23f42;
+  --accent-soft: rgba(168, 85, 247, 0.16);
+  --danger: #ff5a52;
   --online: #23a55a;
 
-  --radius: 10px;
-  --radius-sm: 7px;
-  --sans: 'Inter', system-ui, 'Segoe UI', Roboto, sans-serif;
-  --mono: ui-monospace, 'SF Mono', Consolas, monospace;
+  --radius: 12px;
+  --radius-sm: 8px;
+  --display: 'Major Mono Display', ui-monospace, monospace;
+  --sans: 'IBM Plex Mono', ui-monospace, 'SF Mono', Consolas, monospace;
+  --mono: 'IBM Plex Mono', ui-monospace, 'SF Mono', Consolas, monospace;
 
   color-scheme: dark;
   font-synthesis: none;
@@ -198,6 +202,19 @@ button {
   padding: 32px;
   box-shadow: 0 20px 50px -20px rgba(0, 0, 0, 0.6);
 }
+.brand-wordmark {
+  font-family: var(--display);
+  text-transform: lowercase;
+  font-size: 30px;
+  color: var(--accent);
+  text-shadow: 0 0 32px rgba(168, 85, 247, 0.4);
+  margin-bottom: 18px;
+}
+.brand-logo {
+  max-height: 44px;
+  width: auto;
+  margin-bottom: 18px;
+}
 .login-card h1 {
   font-size: 24px;
   margin-bottom: 4px;
@@ -415,6 +432,20 @@ button {
 .chan-btn.xt-custom {
   --xt: #c084fc;
 }
+.chan-btn.xt-dm {
+  --xt: #a855f7;
+}
+/* x-type group headers in the channel sidebar */
+.dc-chan-group {
+  margin-bottom: 6px;
+}
+.dc-group-label {
+  font-size: 10px;
+  letter-spacing: 0.18em;
+  text-transform: uppercase;
+  color: var(--text-faint);
+  padding: 8px 8px 4px;
+}
 .chan-tag {
   margin-left: auto;
   font-size: 10px;