feat: add option to preserve namespaces when switching contexts

[sarub0b0]

Summary

This PR implements a namespace preservation feature when switching contexts. Users can now maintain their current namespaces across different clusters when the same namespace names exist, enabling smoother transitions without re-selecting namespaces.

Key Changes:

• Add Ctrl+Space option to preserve current namespaces when switching contexts
• Implement intelligent fallback handling when namespaces don't exist in the target context

Implementation Details

Message Layer Extension (src/features/context/message.rs:14-20)

• Add keep_namespace flag to ContextRequest::Set
• Allow specification of whether to preserve namespaces during context switching

UI Layer Improvements (src/features/context/view/dialog.rs)

• Normal selection (Enter): Switch to previously used namespaces in the target context (existing behavior, uses store cache)
• Ctrl+Space: Preserve current namespaces when switching context (new feature)
• Refactored to reduce code duplication by extracting common helper functions

Controller Layer Logic (src/workers/kube/controller.rs:228-250, 588-622)

• When keep_namespace: true, pass current namespace list to the next context
• Verify namespace existence in the new context:
  • All exist: Use them as-is
  • Partial match: Use only available namespaces (respect user intent as much as possible)
  • None exist: Fallback to store cache
• Output appropriate log information for each case (info/warn levels)

User Experience

Users can switch contexts in two ways:

1. Enter: Switch to previously used namespaces in the target context (existing behavior, uses store cache)
2. Ctrl+Space: Preserve current namespaces if they exist, or fallback appropriately (new feature)

Example Use Case

Scenario:

• prod environment: team-a, team-b, team-c, monitoring
• dev environment: team-a, team-b, default

When switching from prod to dev while viewing team-a, team-b, team-c:

• Enter: Switch to previously used namespaces in dev (e.g., default)
• Ctrl+Space: Show only team-a, team-b (team-c is excluded as it doesn't exist, warning logged)

Additional Improvements

Code Quality

• Refactored context dialog to reduce code duplication (~50 lines)
• Extracted common logic into helper functions (clear_widgets_and_close_dialog, switch_context)
• Removed unused imports for cleaner code

Documentation

• Added Context Dialog section to in-app help dialog (? key)
• Updated README.md with Ctrl+Space keybinding documentation
• Documented the difference between Enter and Ctrl+Space behavior

Test plan

• Verify namespace preservation when switching between contexts with the same namespaces using Ctrl+Space
• Verify only available namespaces are shown when switching to a context with partial namespace matches using Ctrl+Space
• Verify fallback to store cache when switching to a context with no matching namespaces using Ctrl+Space
• Verify Enter key switching works as before
• Verify behavior when multiple namespaces are selected

Future Work

When some namespaces are not found, we currently output warnings to logs. In the future, we plan to consider UI notifications as well. (Notification system will be proposed in a separate issue)