feat: Improve UI layout and navigation

- Increase logo size (48x48 desktop, 56x56 mobile) for better visibility
- Add logo as favicon
- Add logo to mobile header
- Move user menu to navigation bars (sidebar on desktop, bottom bar on mobile)
- Fix desktop chat layout - container structure prevents voice controls cutoff
- Fix mobile bottom bar - use icon-only ActionIcons instead of truncated text buttons
- Hide Create Node/New Conversation buttons on mobile to save header space
- Make fixed header and voice controls work properly with containers

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

components/Navigation/MobileHeader.tsx

@@ -0,0 +1,40 @@
+'use client';
+
+/**
+ * Mobile Header
+ *
+ * Fixed header for mobile devices showing the Ponderants logo.
+ */
+
+import { Group, Image, Text, Paper } from '@mantine/core';
+
+export function MobileHeader() {
+  return (
+    <Paper
+      withBorder
+      p="md"
+      radius={0}
+      style={{
+        position: 'fixed',
+        top: 0,
+        left: 0,
+        right: 0,
+        zIndex: 100,
+        borderBottom: '1px solid #dee2e6',
+      }}
+    >
+      <Group gap="sm" align="center">
+        <Image
+          src="/logo.svg"
+          alt="Ponderants logo"
+          w={56}
+          h={56}
+          style={{ flexShrink: 0 }}
+        />
+        <Text fw={700} size="xl">
+          Ponderants
+        </Text>
+      </Group>
+    </Paper>
+  );
+}