Add startup configuration and remove deck creation UI

- Created startup configuration system for automatic relay and account setup
  - New StartupConfig struct with optional relay and nsec fields
  - Load/save functions for startup_config.json
  - Config file location: ~/.local/share/notedeck/settings/startup_config.json

- Integrated startup config into app initialization
  - Automatically adds account from nsec if provided
  - Automatically connects to relay if provided
  - Loads before default relay configuration

- Removed all deck creation functionality from side panel
  - Removed NewDeck, SwitchDeck, and EditDeck actions
  - Removed deck UI elements (add button, deck list)
  - Simplified side panel to show only essential controls
  - Cleaned up unused code (DECK_ICON_SIZE, unused imports)

- Added documentation
  - STARTUP_CONFIG.md with detailed usage instructions
  - startup_config.json.example with configuration template

This allows users to configure their preferred relay and identity once in a
config file rather than having to set it up each time the application starts.

startup_config.json.example

@@ -0,0 +1,4 @@
+{
+  "relay": "wss://relay.damus.io",
+  "nsec": "nsec1your_private_key_here"
+}