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

scripts/demo-init.lua

@@ -1,30 +0,0 @@
-vim.opt.runtimepath:prepend(vim.fn.getcwd())
-local tmpdir = vim.fn.tempname()
-vim.fn.mkdir(tmpdir, 'p')
-
-vim.g.pending = {
-  data_path = tmpdir .. '/tasks.json',
-}
-
-local store = require('pending.store')
-store.load()
-
-local today = os.date('%Y-%m-%d')
-local yesterday = os.date('%Y-%m-%d', os.time() - 86400)
-local tomorrow = os.date('%Y-%m-%d', os.time() + 86400)
-
-store.add({
-  description = 'Finish quarterly report',
-  category = 'Work',
-  due = tomorrow,
-  recur = 'monthly',
-  priority = 1,
-})
-store.add({ description = 'Review pull requests', category = 'Work' })
-store.add({ description = 'Update deployment docs', category = 'Work', status = 'done' })
-store.add({ description = 'Buy groceries', category = 'Personal', due = today })
-store.add({ description = 'Call dentist', category = 'Personal', due = yesterday, priority = 1 })
-store.add({ description = 'Read chapter 5', category = 'Personal' })
-store.add({ description = 'Learn a new language', category = 'Someday' })
-store.add({ description = 'Plan hiking trip', category = 'Someday' })
-store.save()

scripts/demo.tape

@@ -1,28 +0,0 @@
-Output assets/demo.gif
-
-Require nvim
-
-Set Shell "bash"
-Set FontSize 14
-Set Width 900
-Set Height 450
-
-Type "nvim -u scripts/demo-init.lua -c 'autocmd VimEnter * Pending'"
-Enter
-
-Sleep 2s
-
-Down
-Down
-Sleep 300ms
-Down
-Sleep 300ms
-
-Enter
-Sleep 500ms
-
-Tab
-Sleep 1s
-
-Type "q"
-Sleep 200ms