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/_nav.html

@@ -3,8 +3,8 @@
     <div class="nav-content">
         <div class="nav-left">
             <a href="{{ url_for('index') }}" class="logo-section">
-                <img src="{{ url_for('serve_logo') }}" alt="BalanceBoard" class="nav-logo">
-                <span class="brand-text"><span class="brand-balance">balance</span><span class="brand-board">Board</span></span>
+                <img src="{{ url_for('serve_logo') }}" alt="{{ APP_NAME }}" class="nav-logo">
+                <span class="brand-text">{{ APP_NAME }}</span>
             </a>
         </div>