feat(picker): add telescope-style user mappings

Closes #78. Expose `mappings = { i = {...}, n = {...} }` config to bind
arbitrary lhs to function/string rhs on the input buffer, applied after
built-in keymaps.

README.md

@@ -244,6 +244,14 @@ require('fff').setup({
     focus_list = '<leader>l',
     focus_preview = '<leader>p',
   },
+  -- Telescope-style user mappings on the input buffer, keyed by mode (`i` / `n`).
+  -- Values may be a function or a string (rhs). Applied on top of built-in keymaps.
+  mappings = {
+    i = {
+      -- ["<A-BS>"] = function() vim.api.nvim_input("<C-w>") end,
+    },
+    n = {},
+  },
   frecency = {
     enabled = true,
     db_path = vim.fn.stdpath('cache') .. '/fff_nvim',

lua/fff/conf.lua

@@ -69,6 +69,7 @@ local M = {}
 --- @field layout FffLayoutConfig
 --- @field preview FffPreviewConfig
 --- @field keymaps FffKeymapsConfig
+--- @field mappings table<string, table<string, function|string>> User-defined per-mode keymaps. Telescope-style: { i = { ["<A-BS>"] = fn }, n = { ... } }. Mapped on the input buffer.
 --- @field hl table<string, string>
 --- @field frecency FffFrecencyConfig
 --- @field history FffHistoryConfig
@@ -262,6 +263,16 @@ local function init()
       focus_list = '<leader>l',
       focus_preview = '<leader>p',
     },
+    -- Telescope-style user mappings keyed by mode. Mapped on the input buffer.
+    -- Example:
+    --   mappings = {
+    --     i = { ["<A-BS>"] = function() vim.api.nvim_input("<C-w>") end },
+    --     n = { ["<C-r>"] = function() ... end },
+    --   }
+    mappings = {
+      i = {},
+      n = {},
+    },
     hl = {
       border = 'FloatBorder',
       normal = 'Normal',

lua/fff/picker_ui.lua

@@ -1195,6 +1195,13 @@ function M.setup_keymaps()
   set_keymap({ 'i', 'n' }, keymaps.send_to_quickfix, M.send_to_quickfix, input_opts)
   set_keymap({ 'i', 'n' }, keymaps.cycle_grep_modes, M.cycle_grep_modes, input_opts)
 
+  local user_mappings = M.state.config.mappings or {}
+  for _, mode in ipairs({ 'i', 'n' }) do
+    for lhs, rhs in pairs(user_mappings[mode] or {}) do
+      if type(rhs) == 'function' or type(rhs) == 'string' then vim.keymap.set(mode, lhs, rhs, input_opts) end
+    end
+  end
+
   -- List buffer
   set_keymap('n', keymaps.close, M.close, list_opts)
   set_keymap('n', 'q', M.close, list_opts)