M4: percent-complete, conflict-safe saves & subtask reparent

M3 detail/edit polish:
- "Progress" slider on the edit form writes Tasks.PERCENT_COMPLETE (clamped
  0-100, 5% detents); status stays owned by the complete toggle.
- Conflict-safe saves: updateTask re-checks the provider's last_modified against
  the value captured when the form loaded and throws TaskConflictException; the
  editor offers overwrite-or-cancel instead of clobbering an external change.

M4 subtasks (UI):
- Reparent: a "Parent task" picker files a task under any top-level task in its
  list (or "None" to promote it); candidates stay top-level to keep nesting one
  level deep. Switching list clears the now-invalid parent.
- Tapping a subtask opens its own detail (a new TaskDetail entry, own VM).

Tests: percent clamping, parent-id write, populatedFields reveal logic.
Docs: ROADMAP M2/M3/M4 reconciled to the codebase; CHANGELOG [Unreleased].

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/res/values/strings.xml

@@ -29,9 +29,14 @@
 
     <!-- Task list: inline add + swipe -->
     <string name="add_task_hint">Add a task</string>
+    <string name="add_subtask_hint">Add a subtask</string>
     <string name="cd_add_task">Add task</string>
     <string name="cd_complete">Complete</string>
     <string name="cd_reopen">Reopen</string>
+    <string name="task_deleted">Deleted</string>
+    <string name="undo">Undo</string>
+    <string name="cd_expand_subtasks">Show subtasks</string>
+    <string name="cd_collapse_subtasks">Hide subtasks</string>
 
     <!-- Sections (due-date buckets) -->
     <string name="section_overdue">Overdue</string>
@@ -55,6 +60,8 @@
     <string name="detail_priority">Priority</string>
     <string name="detail_progress">Progress</string>
     <string name="detail_subtasks">Subtasks</string>
+    <string name="detail_parent">Parent task</string>
+    <string name="detail_part_of">Part of</string>
     <string name="percent_complete">%1$d%%</string>
 
     <!-- Task edit -->
@@ -63,6 +70,11 @@
     <string name="edit_due_label">Due</string>
     <string name="edit_all_day">All day</string>
     <string name="edit_priority_label">Priority</string>
+    <string name="edit_progress_label">Progress</string>
+    <string name="edit_parent_label">Parent task</string>
+    <string name="edit_parent_none">None (top-level)</string>
+    <string name="edit_parent_search">Search tasks</string>
+    <string name="edit_parent_empty">No matching tasks in this list.</string>
     <string name="edit_reminder_label">Reminder</string>
     <string name="edit_set">Set</string>
     <string name="edit_clear">Clear</string>
@@ -72,6 +84,11 @@
     <string name="close">Close</string>
     <string name="dialog_cancel">Cancel</string>
 
+    <!-- Save conflict -->
+    <string name="edit_conflict_title">Task changed elsewhere</string>
+    <string name="edit_conflict_body">This task was updated somewhere else (a sync or another app) since you opened it. Overwrite those changes with your edits?</string>
+    <string name="edit_conflict_overwrite">Overwrite</string>
+
     <!-- Reminder offsets -->
     <string name="reminder_none">None</string>
     <string name="reminder_at_due">At time of task</string>