feat: overdue highlighting, relative dates, undo write, buffer mappings (#1)

* feat(config): add category_order field

Problem: category display order was always insertion order with no way
to configure it.

Solution: add category_order to config defaults so users can declare a
preferred category ordering; unspecified categories append after.

* feat(parse): add relative date resolution

Problem: due dates required full YYYY-MM-DD input, adding friction for
common cases like "today" or "next monday".

Solution: add resolve_date() supporting today, tomorrow, +Nd, and
weekday abbreviations; extend inline token parsing to resolve relative
values before falling back to strict date validation.

* feat(views): overdue flag, category in priority view, category ordering

Problem: overdue tasks were visually indistinct from upcoming ones;
priority view had no category context; category display order was not
configurable.

Solution: compute overdue meta flag for pending tasks past their due
date; set show_category on priority view task meta; reorder categories
according to config.category_order when present.

* feat(buffer): overdue highlight, category virt text in priority view

Problem: overdue tasks had no visual distinction; priority view showed
no category context alongside due dates.

Solution: add PendingOverdue highlight group; render category name as
right-aligned virtual text in priority view, composited with the due
date when both are present.

* feat(init): undo write and buffer-local default mappings

Problem: _undo_state was captured on every save but never consumed;
toggle_priority and prompt_date had no buffer-local defaults, requiring
manual <Plug> configuration.

Solution: implement undo_write() to restore pre-save task state; add !,
d, and U as buffer-local defaults following fugitive's philosophy of
owning the buffer; expose :Pending undo as a command alias.

* test(views): add views spec

Problem: views.lua had no test coverage.

Solution: add 26 tests covering category_view and priority_view
including sort order, line format, overdue detection, show_category
meta, and category_order config behavior.

* test(archive): add archive spec

Problem: archive had no test coverage.

Solution: add 9 tests covering cutoff logic, custom day counts, pending
task preservation, deleted task cleanup, and notify output.

* docs: add vimdoc

Problem: no :help documentation existed.

Solution: add doc/pending.txt covering all features — commands,
mappings, views, configuration, Google Calendar sync, highlight groups,
data format, and health check — following standard vimdoc conventions.

* ci: format

* fix: resolve lint and type check errors

Problem: selene flagged unused variables in new spec files; LuaLS
flagged os.date/os.time return type mismatches, integer? assignments,
and stale task.Task/task.GcalConfig type references.

Solution: prefix unused spec variables with _ or drop unnecessary
assignments; add --[[@as string/integer]] casts for os.date and
os.time calls; add category_order field to pending.Config annotation;
fix task.GcalConfig -> pending.GcalConfig and task.Task[] ->
pending.Task[]; add nil guards on meta[row].id before store calls;
cast store.data() return to non-optional.

* ci: format

* fix: sync

* ci: format

lua/pending/buffer.lua

@@ -88,9 +88,25 @@ local function apply_extmarks(bufnr, line_meta)
   for i, m in ipairs(line_meta) do
     local row = i - 1
     if m.type == 'task' then
-      if m.due then
+      local due_hl = m.overdue and 'PendingOverdue' or 'PendingDue'
+      if m.show_category then
+        local virt_text
+        if m.category and m.due then
+          virt_text = { { m.category .. '  ', 'PendingHeader' }, { m.due, due_hl } }
+        elseif m.category then
+          virt_text = { { m.category, 'PendingHeader' } }
+        elseif m.due then
+          virt_text = { { m.due, due_hl } }
+        end
+        if virt_text then
+          vim.api.nvim_buf_set_extmark(bufnr, task_ns, row, 0, {
+            virt_text = virt_text,
+            virt_text_pos = 'right_align',
+          })
+        end
+      elseif m.due then
         vim.api.nvim_buf_set_extmark(bufnr, task_ns, row, 0, {
-          virt_text = { { m.due, 'PendingDue' } },
+          virt_text = { { m.due, due_hl } },
           virt_text_pos = 'right_align',
         })
       end
@@ -120,6 +136,7 @@ local function setup_highlights()
   end
   hl('PendingHeader', { bold = true })
   hl('PendingDue', { fg = '#888888', italic = true })
+  hl('PendingOverdue', { fg = '#e06c75', italic = true })
   hl('PendingDone', { strikethrough = true, fg = '#666666' })
   hl('PendingPriority', { fg = '#e06c75', bold = true })
 end