feat: :Pending auth subcommands + fix #61 (#84)

* fix(buffer): use `default_category` config for empty placeholder

Problem: The empty-buffer fallback hardcoded the category name `TODO`,
ignoring the user's `default_category` config value (default: `Todo`).

Solution: Read `config.get().default_category` at render time and use
that value for both the header line and `LineMeta` category field.

* fix(diff): match optional checkbox char in `parse_buffer` patterns

Problem: `parse_buffer` used `%[.%]` which requires exactly one
character between brackets, failing to parse empty `[]` checkboxes.

Solution: Change to `%[.?%]` so the character is optional, matching
`[]`, `[ ]`, `[x]`, and `[!]` uniformly.

* fix(init): add `nowait` to buffer keymap opts

Problem: Buffer-local mappings like `!` could be swallowed by Neovim's
operator-pending machinery or by global maps sharing a prefix, since
the keymap opts did not include `nowait`.

Solution: Add `nowait = true` to the shared `opts` table used for all
buffer-local mappings in `_setup_buf_mappings`.

* feat(init): allow `:Pending done` with no args to use cursor line

Problem: `:Pending done` required an explicit task ID, making it
awkward to mark the current task done while inside the pending buffer.

Solution: When called with no ID, `M.done()` reads the cursor row from
`buffer.meta()` to resolve the task ID, erroring if the cursor is not
on a saved task line.

* fix(views): populate `priority` field in `LineMeta`

Problem: Both `category_view` and `priority_view` omitted `priority`
from the `LineMeta` they produced. `apply_extmarks` checks `m.priority`
to decide whether to render the priority icon, so it was always nil,
causing the `[ ]` pending-icon overlay to replace the `[!]` buffer text.

Solution: Add `priority = task.priority` to both LineMeta constructors.

* fix(buffer): keep `_meta` in sync when `open_line` inserts a new line

Problem: `open_line` inserted a buffer line without updating `_meta`,
leaving the entry at that row pointing to the task that was shifted
down. Pressing `<CR>` (toggle_complete) would read the stale meta,
find a real task ID, toggle it, and re-render — destroying the unsaved
new line.

Solution: Insert a `{ type = 'blank' }` sentinel into `_meta` at the
new line's position so buffer-local actions see no task there.

* fix(buffer): use task sentinel in `open_line` for better unsaved-task errors

* feat(init): warn on dirty buffer before store-dependent actions

Problem: `toggle_complete`, `toggle_priority`, `prompt_date`, and
`done` (no-args) all read from `buffer.meta()` which is stale whenever
the buffer has unsaved edits, leading to silent no-ops or acting on the
wrong task.

Solution: Add a `require_saved()` guard that emits a `log.warn` and
returns false when the buffer is modified. Each store-dependent action
calls it before touching meta or the store.

* fix(init): guard `view`, `undo`, and `filter` against dirty buffer

Problem: `toggle_view`, `undo_write`, and `filter` all call
`buffer.render()` which rewrites the buffer from the store, silently
discarding any unsaved edits. The previous `require_saved()` change
missed these three entry points.

Solution: Add `require_saved()` to the `view` and `filter` keymap
lambdas and to `M.undo_write()`. Also guard `M.filter()` directly so
`:Pending filter` from the command line is covered too.

* fix(init): improve dirty-buffer warning message

* fix(init): tighten dirty-buffer warning message

* feat(oauth): add `OAuthClient:clear_tokens()` method

Problem: no way to wipe just the token file while keeping credentials
intact — `_wipe()` removed both.

Solution: add `clear_tokens()` that removes only the token file.

* fix(sync): warn instead of auto-reauth when token is missing

Problem: `with_token` silently triggered an OAuth browser flow when no
tokens existed, with no user-facing explanation.

Solution: replace the auto-reauth branch with a `log.warn` directing
the user to run `:Pending auth`.

* feat(init): add `clear` and `reset` actions to `:Pending auth`

Problem: no CLI path existed to wipe stale tokens or reset credentials,
and the `vim.ui.select` backend picker was misleading given shared tokens.

Solution: accept an args string in `M.auth()`, dispatching `clear` to
`clear_tokens()`, `reset` to `_wipe()`, and bare backend names to the
existing auth flow. Remove the picker.

* feat(plugin): add tab completion for `:Pending auth` subcommands

`:Pending auth <Tab>` completes `gcal gtasks clear reset`;
`:Pending auth <backend> <Tab>` completes `clear reset`.

lua/pending/sync/gcal.lua

@@ -134,16 +134,7 @@ local function with_token(callback)
   oauth.async(function()
     local token = oauth.google_client:get_access_token()
     if not token then
-      oauth.google_client:auth(function()
-        oauth.async(function()
-          local fresh = oauth.google_client:get_access_token()
-          if fresh then
-            callback(fresh)
-          else
-            log.error(oauth.google_client.name .. ': authorization failed or was cancelled')
-          end
-        end)
-      end)
+      log.warn('not authenticated — run :Pending auth')
       return
     end
     callback(token)