docs: document recurrence, expanded dates, omnifunc, new config

Problem: the vimdoc does not cover recurrence, expanded date syntax,
omnifunc completion, or the new config fields.

Solution: add DATE INPUT and RECURRENCE sections, update INLINE
METADATA, COMMANDS, CONFIGURATION, HIGHLIGHT GROUPS, HEALTH CHECK,
and DATA FORMAT. Expand the help popup with recurrence patterns and
new date tokens. Add recurrence validation to healthcheck.

lua/pending/health.lua

@@ -27,6 +27,19 @@ function M.check()
       if load_ok then
         local tasks = store.tasks()
         vim.health.ok('Data file loaded: ' .. #tasks .. ' tasks')
+        local recur = require('pending.recur')
+        local invalid_count = 0
+        for _, task in ipairs(tasks) do
+          if task.recur and not recur.validate(task.recur) then
+            invalid_count = invalid_count + 1
+            vim.health.warn(
+              'Task ' .. task.id .. ' has invalid recurrence spec: ' .. task.recur
+            )
+          end
+        end
+        if invalid_count == 0 then
+          vim.health.ok('All recurrence specs are valid')
+        end
       else
         vim.health.error('Failed to load data file: ' .. tostring(err))
       end