Timers: admin "Timer font" option with a monospace choice

A new Display-group select (Default / Monospace) sets the font for the
timer and LTC digits. Monospace keeps the numbers from shifting sideways
as they change — easier to read on a fast-moving countdown or timecode.
Flows through server state → client applies a `tm-mono` class and re-fits
the digit sizing; both timer and LTC values (.tm-time) honor it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -176,9 +176,10 @@ below. Nothing is installed without someone clicking.
   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 is in
-  two groups: **ProPresenter** (endpoint host : port, API password) and
+  three groups: **ProPresenter** (endpoint host : port, API password),
   **LTC Timecode** (the listener switch, and — when it's on — the audio
-  device and channel).
+  device and channel), and **Display** (Timer font — Default or Monospace,
+  which keeps the digits from shifting as the numbers change).
 
   ProPresenter exposes no timecode over any API, so LTC is decoded from the
   audio signal, behind the admin **LTC listener** switch (off ⇒ no LTC tile