feat: allow editing the "Created by" user on a task

[parth0025]

The task sidebar Details panel previously showed Created by (the Task_Leader field) as a read-only user — set once at creation and never changeable. Add an inline picker mirroring the existing Assignee field so the creator can be reassigned.

Changes:

• Backend: new updateTaskLeader action on task_class_Mongo. Validates the new Task_Leader, writes via $set, emits the same socketEmitter.emit('update', { module: 'task', updatedFields }) used by updateAssignee, and logs a TaskLeader_Changed history entry via HandleHistory. Includes the isUpdateTask:false side-effects-only branch for parity with updateStatus / updatePriority.
• Frontend store (TaskOperations): new updateTaskLeader action that optimistically commits the new Task_Leader into the projectData Vuex store, then PATCHes /api/v2/tasks with action 'updateTaskLeader'.
• TaskDetailRightSide.vue: replaced the read-only Created-by block with an Assignee picker (single-select) for users holding the task.task_assignee + task.task_list permissions, preserving the original read-only display as the fallback for users without those permissions. Wired @selected to a new updateTaskLeader() handler with the same toast / error pattern as updateAssignee.
• i18n: added Toast.Created_by_updated_successfully and Toast.Created_by_not_updated to the English locale. The other 10 locales fall back to English via vue-i18n until translators backfill — flagged as a follow-up.

Permission policy: reuses task.task_assignee. Anyone allowed to change the Assignee can also change Created by. No new permission key, no role-permission migration. Backend has no per-field gate, consistent with the existing Assignee / Status / Priority update paths which all trust the frontend permission gate.

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