fix(init): guard view, undo, and filter against dirty buffer

Problem: `toggle_view`, `undo_write`, and `filter` all call
`buffer.render()` which rewrites the buffer from the store, silently
discarding any unsaved edits. The previous `require_saved()` change
missed these three entry points.

Solution: Add `require_saved()` to the `view` and `filter` keymap
lambdas and to `M.undo_write()`. Also guard `M.filter()` directly so
`:Pending filter` from the command line is covered too.

lua/pending/init.lua

@@ -185,6 +185,9 @@ end
 ---@param pred_str string
 ---@return nil
 function M.filter(pred_str)
+  if not require_saved() then
+    return
+  end
   if pred_str == 'clear' or pred_str == '' then
     buffer.set_filter({}, {})
     local bufnr = buffer.bufnr()
@@ -253,6 +256,9 @@ function M._setup_buf_mappings(bufnr)
       M.toggle_complete()
     end,
     view = function()
+      if not require_saved() then
+        return
+      end
       buffer.toggle_view()
     end,
     priority = function()
@@ -265,6 +271,9 @@ function M._setup_buf_mappings(bufnr)
       M.undo_write()
     end,
     filter = function()
+      if not require_saved() then
+        return
+      end
       vim.ui.input({ prompt = 'Filter: ' }, function(input)
         if input then
           M.filter(input)
@@ -380,6 +389,9 @@ end
 
 ---@return nil
 function M.undo_write()
+  if not require_saved() then
+    return
+  end
   local s = get_store()
   local stack = s:undo_stack()
   if #stack == 0 then