Clean up Aspire panel: context menus, loading state, welcome messages

[adamint]

Description

General cleanup for panel, including requested changes from #15347.

Adds an initial 'loading' screen so that the initial message displayed in the panel is correct. "Searching for running apphosts..."

New resource node context menu:

New link node context menu:

New apphost (global mode only, because the data is not available via aspire describe) context menu:

New apphost pid/cli pid context menus:

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
  • No
• Does the change require an update in our Aspire docs?
  • Yes
  • No