feat: Redesign UI to match Nextcloud ecosystem aesthetic

This commit updates the web interface to better align with Nextcloud's
design system and improve the Vector Viz layout.

Changes:
- Replace emoji icons with Material Design SVG icons for better
  consistency with Nextcloud apps
- Simplify navigation styling with minimal padding and subtle active
  states (250px width)
- Update CSS variables to match Nextcloud design system
- Restructure Vector Viz from two-column to single-column vertical
  layout for better plot visibility
- Move search controls to compact horizontal grid at top
- Make navigation toggle always visible (not just on mobile)
- Fix plot container sizing with overflow:visible to prevent colorbar
  clipping
- Remove heavy shadows and custom card styling for cleaner aesthetic
- Add error and success page templates with consistent styling

Technical details:
- Preserve Alpine.js for reactive functionality
- Use CSS Grid for responsive horizontal controls layout
- Add smooth transitions for navigation collapse/expand
- Maintain HTMX for dynamic content loading

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

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

nextcloud_mcp_server/auth/templates/error.html

@@ -0,0 +1,19 @@
+{% extends "base.html" %}
+
+{% block title %}{{ error_title|default('Error') }} - Nextcloud MCP Server{% endblock %}
+
+{% block content %}
+<h1>{{ error_title|default('Error') }}</h1>
+
+<div class="error">
+    <strong>Error:</strong> {{ error_message }}
+</div>
+
+{% if login_url %}
+<p><a href="{{ login_url }}" class="btn btn-primary">Login again</a></p>
+{% endif %}
+
+{% if back_url %}
+<p><a href="{{ back_url }}" class="btn">Go Back</a></p>
+{% endif %}
+{% endblock %}