Timers: present each timer and LTC in the Add-tile picker

Adopt main's new multi-tile contract: the module's server exports
tiles() (answered from the poller's held state, never a fresh upstream
request) listing All timers, one entry per discovered timer, and LTC
timecode. The client branches on moduleApi.variant — solo tiles render
exactly one card with no selection menu, and a solo timer that vanishes
from ProPresenter shows a labeled "Not in ProPresenter" card instead of
erroring (visible even in the compact layout). pp-timers-mock gains
--drop=<uuid,...> to simulate a timer deleted in ProPresenter.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -88,9 +88,11 @@ are written by the app itself.
   driven by the shared `propresenter-core` client (ProPresenter 7/20).
   Admin config: host, port, password.
 - **Timers** — live ProPresenter timers (countdown, countdown-to-time,
-  elapsed) and LTC timecode as auto-sized cards; each tile picks which to
-  show via its "Timers ▾" menu, overruns are unmistakably red. Read-only —
-  no start/stop controls. Admin config: host, port, password.
+  elapsed) and LTC timecode as auto-sized cards. The ＋ picker lists every
+  discovered timer (and the LTC card) as its own tile, or add "All timers"
+  and pick a selection via the tile's "Timers ▾" menu; overruns are
+  unmistakably red. Read-only — no start/stop controls. Admin config: host,
+  port, password.
 - **Clock** — a big booth clock; also the smallest possible module and the
   reference for module authors.