M3: rebuild task create/edit form in Calendula's card language

Replace the boxed-text-field form with the family's tonal-card layout:
Close + filled SAVE top bar, a borderless headline title with a
list-colour accent bar, a "When" card (all-day switch + tappable
start/due rows), a tappable list card and reminder card that open
OptionCard dialogs, and segmented-button priority.

Optional Description / Priority / Reminder sections unfold from a
"More fields" picker (Calendula's disclosure): TaskFormField enum +
populatedFields() auto-reveal fields that already carry a value, and a
persisted defaultEditFields pref decides which start open (the settings
UI to edit it lands with M6). New shared InlineTextField + OptionCard;
date/time conversion helpers in DateTimeField promoted to internal.

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

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

@@ -66,6 +66,11 @@
     <string name="edit_reminder_label">Reminder</string>
     <string name="edit_set">Set</string>
     <string name="edit_clear">Clear</string>
+    <string name="edit_title_hint">Task title</string>
+    <string name="edit_when">When</string>
+    <string name="edit_more_fields">More fields</string>
+    <string name="close">Close</string>
+    <string name="dialog_cancel">Cancel</string>
 
     <!-- Reminder offsets -->
     <string name="reminder_none">None</string>