Add configurable logo and application name support

Added environment variables APP_NAME and LOGO_PATH to make the application
branding configurable:

- APP_NAME (default: 'BalanceBoard'): Sets the application name
- LOGO_PATH (default: 'logo.png'): Sets the logo file path

Changes:
- Added configuration variables to app.py
- Updated logo serving route to support custom paths
- Added template context processor to inject APP_NAME
- Updated all templates to use {{ APP_NAME }} instead of hardcoded 'BalanceBoard'
- Updated navigation and branding to use configurable values

Users can now customize their installation by setting:
export APP_NAME="My Custom Board"
export LOGO_PATH="/path/to/my/logo.png"

Fixes #22

~claude

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

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

templates/password_reset.html

@@ -1,12 +1,12 @@
 {% extends "base.html" %}
 
-{% block title %}Set New Password - BalanceBoard{% endblock %}
+{% block title %}Set New Password - {{ APP_NAME }}{% endblock %}
 
 {% block content %}
 <div class="auth-container">
     <div class="auth-card">
         <div class="auth-logo">
-            <img src="{{ url_for('serve_logo') }}" alt="BalanceBoard Logo">
+            <img src="{{ url_for('serve_logo') }}" alt="{{ APP_NAME }} Logo">
             <h1><span class="balance">balance</span>Board</h1>
             <p style="color: var(--text-secondary); margin-top: 8px;">Set a new password</p>
         </div>