feat: allow configuring default resource per service in config file

[yimsk]

Summary

Allow users to override the default resource type for a service via config file.

Use Case

When accessing a service without specifying a resource type (e.g., :ec2 or claws -s ec2), claws uses a sensible default (e.g., instances). Some users may prefer different defaults based on their workflow.

Proposed Config

startup:
  defaults:
    ec2: volumes      # override built-in "instances"
    sns: topics       # override alphabetical fallback

Implementation Notes

• SetDefaultResource() already exists in registry.go
• User-configured defaults take precedence over built-in defaults
• Built-in defaults fall back to alphabetically first if not configured

Related

• Part of Feature Proposal: Start TUI Focusing On A Service/Resource #97 (startup service navigation)