fix: harden sync backends and fix edit recompute (#66)

* refactor(oauth): async coroutine support, pure-Lua PKCE, server hardening

Problem: OAuth module shelled out to openssl for PKCE, used blocking
`vim.system():wait()`, had a weak `os.time()` PRNG seed, and the TCP
callback server leaked on read errors with no timeout.

Solution: Add `M.system()` coroutine wrapper and `M.async()` helper,
replace openssl with `vim.fn.sha256` + `vim.base64.encode`, seed from
`vim.uv.hrtime()`, add `close_server()` guard with 120s timeout, and
close the server on read errors.

* fix(gtasks): async operations, error notifications, buffer refresh

Problem: Sync operations blocked the editor, `push_pass` silently
dropped delete/update/create API errors, and the buffer was not
re-rendered after push/pull/sync.

Solution: Wrap `push`, `pull`, `sync` in `oauth.async()`, add
`vim.notify` for all `push_pass` failure paths, and re-render the
pending buffer after each operation.

* fix(init): edit recompute, filter predicates, sync action listing

Problem: `M.edit()` skipped `_recompute_counts()` after saving,
`compute_hidden_ids` lacked `done`/`pending` predicates, and
`run_sync` defaulted to `sync` instead of listing available actions.

Solution: Replace `s:save()` with `_save_and_notify()` in `M.edit()`,
add `done` and `pending` filter predicates, and list backend actions
when no action is specified.

* refactor(gcal): per-category calendars, async push, error notifications

Problem: gcal used a single hardcoded calendar name, ran synchronously
blocking the editor, and silently dropped some API errors.

Solution: Fetch all calendars and map categories to calendars (creating
on demand), wrap push in `oauth.async()`, notify on individual API
failures, track `_gcal_calendar_id` in `_extra`, and remove the `$`
anchor from `next_day` pattern.

* refactor: formatting fixes, config cleanup, health simplification

Problem: Formatter disagreements in `init.lua` and `gtasks.lua`,
stale `calendar` field in gcal config, and redundant health checks
for data directory existence.

Solution: Apply stylua formatting, remove `calendar` field from
`pending.GcalConfig`, drop data-dir and no-file health messages,
add `done`/`pending` to filter tab-completion candidates.

* docs: update vimdoc for sync refactor, remove demo scripts

Problem: Docs still referenced openssl dependency, defaulting to `sync`
action, and the `calendar` config field. Demo scripts used the old
singleton `store` API.

Solution: Update vimdoc and README to reflect explicit actions, per-
category calendars, and pure-Lua PKCE. Remove stale demo scripts and
update sync specs to match new behavior.

* fix(types): correct LuaLS annotations in oauth and gcal

lua/pending/health.lua

@@ -25,13 +25,6 @@ function M.check()
     vim.health.info('(project-local store; global path: ' .. cfg.data_path .. ')')
   end
 
-  local data_dir = vim.fn.fnamemodify(resolved_path, ':h')
-  if vim.fn.isdirectory(data_dir) == 1 then
-    vim.health.ok('Data directory exists: ' .. data_dir)
-  else
-    vim.health.warn('Data directory does not exist yet: ' .. data_dir)
-  end
-
   if vim.fn.filereadable(resolved_path) == 1 then
     local s = store.new(resolved_path)
     local load_ok, err = pcall(function()
@@ -54,8 +47,6 @@ function M.check()
     else
       vim.health.error('Failed to load data file: ' .. tostring(err))
     end
-  else
-    vim.health.info('No data file yet (will be created on first save)')
   end
 
   local sync_paths = vim.fn.globpath(vim.o.runtimepath, 'lua/pending/sync/*.lua', false, true)