Fix Mission dialog title reset and add unsaved-changes warning

[Knobee]

Summary

• Addresses Issues 1 and 2 of Mission Dialog UX Improvements #180.
• Issue 1: After editing a mission, clicking "New Mission" again now correctly shows "Add Mission" as the dialog title and submit button label, instead of the stale "Edit Mission".
• Issue 2: Closing the mission dialog (outside-click, ESC, or close button) with unsaved changes now opens a confirmation prompt instead of silently discarding the work. Saving or discarding clears the dirty flag.

Issue 3 — out of scope

Issue 3 (no UI to remove an existing mission password) needs a backend change before it can be done cleanly:

• The DELETE /Marti/api/missions/<name>/password endpoint cited in the issue requires a Data Sync token / iTAK certificate and is not callable from the admin web UI.
• The /api/missions POST handler ignores password: "" on create (mission_api.py:103).
• The PUT/edit branch would set password="" but does not recompute password_protected, leaving stale state.

This needs a separate UI + backend PR.

Test plan

• Open "New Mission", close, click Edit on a mission, close, click "New Mission" — dialog title and submit button both read "Add Mission".
• Open "New Mission", type into Name, press ESC — confirmation prompt appears. "Discard" closes both dialogs; "Keep editing" only closes the prompt.
• Open "New Mission", click outside dialog without typing anything — closes silently (no false-positive prompt).
• Submitting a new or edited mission via the dialog button still works and clears the dirty flag.

🤖 Generated with Claude Code

[brian7704]

Thanks for this PR. I tested it but the new "are you sure" modal renders behind the already opened add/edit mission modal making it unclickable.