fix(diff): preserve due/rec when absent from buffer line

Problem: `diff.apply` overwrites `task.due` and `task.recur` with `nil`
whenever those fields aren't present as inline tokens in the buffer line.
Because metadata is rendered as virtual text (never in the line text),
every description edit silently clears due dates and recurrence rules.

Solution: Only update `due`, `recur`, and `recur_mode` in the existing-
task branch when the parsed entry actually contains them (non-nil). Users
can still set/change these inline by typing `due:<date>` or `rec:<rule>`;
clearing them requires `:Pending edit <id> -due`.

lua/pending/diff.lua

@@ -121,17 +121,19 @@ function M.apply(lines, s, hidden_ids)
           task.priority = entry.priority
           changed = true
         end
-        if task.due ~= entry.due then
+        if entry.due ~= nil and task.due ~= entry.due then
           task.due = entry.due
           changed = true
         end
-        if task.recur ~= entry.rec then
-          task.recur = entry.rec
-          changed = true
-        end
-        if task.recur_mode ~= entry.rec_mode then
-          task.recur_mode = entry.rec_mode
-          changed = true
+        if entry.rec ~= nil then
+          if task.recur ~= entry.rec then
+            task.recur = entry.rec
+            changed = true
+          end
+          if task.recur_mode ~= entry.rec_mode then
+            task.recur_mode = entry.rec_mode
+            changed = true
+          end
         end
         if entry.status and task.status ~= entry.status then
           task.status = entry.status