feat(buffer): persist fold state across sessions

Problem: folded category headers are lost when Neovim exits because
`_fold_state` only lives in memory. Users must re-fold categories
every session.

Solution: store folded category names in the JSON data file as a
top-level `folded_categories` field. On first render, `restore_folds`
seeds from the store instead of the empty in-memory state. Folds are
persisted on `M.close()` and `VimLeavePre`.

lua/pending/store.lua

@@ -20,6 +20,7 @@ local config = require('pending.config')
 ---@field next_id integer
 ---@field tasks pending.Task[]
 ---@field undo pending.Task[][]
+---@field folded_categories string[]
 
 ---@class pending.Store
 ---@field path string
@@ -39,6 +40,7 @@ local function empty_data()
     next_id = 1,
     tasks = {},
     undo = {},
+    folded_categories = {},
   }
 end
 
@@ -171,6 +173,7 @@ function Store:load()
     next_id = decoded.next_id or 1,
     tasks = {},
     undo = {},
+    folded_categories = decoded.folded_categories or {},
   }
   for _, t in ipairs(decoded.tasks or {}) do
     table.insert(self._data.tasks, table_to_task(t))
@@ -199,6 +202,7 @@ function Store:save()
     next_id = self._data.next_id,
     tasks = {},
     undo = {},
+    folded_categories = self._data.folded_categories,
   }
   for _, task in ipairs(self._data.tasks) do
     table.insert(out.tasks, task_to_table(task))
@@ -371,6 +375,17 @@ function Store:set_next_id(id)
   self:data().next_id = id
 end
 
+---@return string[]
+function Store:get_folded_categories()
+  return self:data().folded_categories
+end
+
+---@param cats string[]
+---@return nil
+function Store:set_folded_categories(cats)
+  self:data().folded_categories = cats
+end
+
 ---@return nil
 function Store:unload()
   self._data = nil