Allow k9s to start without requiring a valid Kubernetes context

[uozalp]

For users managing multiple Kubernetes clusters, the current behavior of k9s creates friction:

• You have to remember to run kubectl config use-context <cluster>before starting k9s
• If you have a work cluster set as current context but start k9s at home (without VPN), k9s hangs for minutes trying to connect before failing
• When the configured context is unreachable, k9s exits with errors instead of allowing you to select a different context

# At home
kubectl config use-context homelab
k9s

# At work (or home after connecting to VPN)
kubectl config use-context work-cluster 
k9s

# Back at home
kubectl config use-context homelab  # If you forget this, k9s will hang trying to connect to work cluster
k9s

Proposed Solution

• If no context is set, show the context view immediately
• Let users select the appropriate context from within k9s
• Eliminate the manual kubectl config use-context step

PR https://github.com/derailed/k9s/pull/3704