fix: dialog/store correctness — probe-removal, race, owner gate (#260 #261 #262 #264)

[rubenvdlinde]

Summary

• [MEDIUM] ProjectLeaveDialog removes user as a probe before confirmation — Cancel button is a lie #260 ProjectLeaveDialog probe mutation: mounted() no longer calls leaveProject as a probe. It fetches the project and inspects the members array client-side — Cancel can no longer mutate server state.
• [MEDIUM] confirmRemoveMember removes-then-re-adds as probe — member silently lost if re-add fails #261/[MEDIUM] Read-modify-write race on members[] — concurrent addMember/removeMember silently drops updates #262 confirmRemoveMember remove-then-re-add race: removeMember is now a pure write action. A new getMemberTaskCount read action handles the count query. confirmRemoveMember calls the read action first and only removes when count is zero, eliminating the remove-then-re-add race and silent data loss if re-add fails.
• [MEDIUM] ProjectDeleteDialog has no admin/creator gate and no confirmation safeguard #264 ProjectDeleteDialog no admin/creator gate: Added canDelete computed (owner === currentUid || isAdmin) that gates the delete button and confirm() server-side guard. createProject now sets the owner field so newly created projects are always gated correctly.

Test plan

• Open Leave Project dialog — verify Cancel does not modify membership
• Remove a member with assigned tasks — verify remove-then-re-add no longer happens (member stays until "Remove anyway" is clicked)
• Delete a project as non-owner/non-admin — verify delete button is disabled and shows forbidden message
• Delete a project as owner — verify delete works normally
• Create a new project and verify owner field is set to current user's UID