feat(config): add keymaps table for buffer-local bindings

Problem: Users had no way to customize or disable buffer-local key
bindings in the pending buffer.

Solution: Add a pending.Keymaps class and keymaps field to
pending.Config with defaults for all eight buffer actions. Setting any
key to false disables that binding.

lua/pending/config.lua

@@ -2,6 +2,16 @@
 ---@field calendar? string
 ---@field credentials_path? string
 
+---@class pending.Keymaps
+---@field close? string|false
+---@field toggle? string|false
+---@field view? string|false
+---@field priority? string|false
+---@field date? string|false
+---@field undo? string|false
+---@field open_line? string|false
+---@field open_line_above? string|false
+
 ---@class pending.Config
 ---@field data_path string
 ---@field default_view 'category'|'priority'
@@ -12,6 +22,7 @@
 ---@field someday_date string
 ---@field category_order? string[]
 ---@field drawer_height? integer
+---@field keymaps pending.Keymaps
 ---@field gcal? pending.GcalConfig
 
 ---@class pending.config
@@ -27,6 +38,16 @@ local defaults = {
   recur_syntax = 'rec',
   someday_date = '9999-12-30',
   category_order = {},
+  keymaps = {
+    close = 'q',
+    toggle = '<CR>',
+    view = '<Tab>',
+    priority = '!',
+    date = 'D',
+    undo = 'U',
+    open_line = 'o',
+    open_line_above = 'O',
+  },
 }
 
 ---@type pending.Config?