fix: minor label consistency and wording updates

[danbarr]

Some small updates to labels:

• Fixed some capitalization inconsistencies
• Made the custom server submission button label consistent with adding from registry ("Install server", not "Submit")
• Added :latest / @latest to example image/package inputs
• On the update secret modal, updated some strings that didn't sound right to me:
  • "Replacement secret" -> "New value"
  • "Secret contents" -> "Secret value