Stopwatch counts in real time from a start date, kept over a restart

The stopwatch counted with millis(), so it started at 0 after every
restart and would overflow after 49 days. It now keeps the real start
time (UTC ms) and the time counted before a pause, stored in NVS
(system/stopwatch_store) on every change, so it keeps counting while
the display is off or restarting. Stop pauses, Start continues, and the
Stopwatch tab can set the start to a date and time (local, summer time
by mktime) to count from, e.g. days since an event. Start, stop and set
need the synced time.

New page formats: 12d 4:05:09 and 3w 2d.

Version 2.19.0, tested on the display over OTA (set start, future and
invalid dates refused, pause, restart).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/web-ui.md

@@ -36,7 +36,7 @@ pages that can be changed; their Reset button brings back the original. See the
 | Home Assistant value | An entity or attribute (also a path into lists, e.g. `forecast.1.temperature`), text before and after. Shown as a number (multiplier, decimals, auto unit `1257 W` → `1.3 kW`, without minus sign), a duration (`2h 21m`, `2:21:00`, `2:21`, `2 uur`), a time from an HA timestamp (`23:07`, `do 23:07`, `15-10 23:07`, `over 34 min`) or with own texts per state (`printing=Bezig, standby=Klaar`) |
 | Text | Up to 32 characters |
 | Countdown | To a date and time: smart (`12 dagen`, `5 uur`, `34 min`, `0:45`), days, `12d 04:05`, with seconds; a text when it has passed |
-| Stopwatch | `4:05`, `0:04`, `0:04:05`; started in the [Stopwatch](#stopwatch) tab or Home Assistant |
+| Stopwatch | `4:05`, `0:04`, `0:04:05`, `12d 4:05:09`, `3w 2d`; started in the [Stopwatch](#stopwatch) tab or Home Assistant |
 | Image | An icon (or a bitmap) at a position, fixed or chosen by a Home Assistant state (`weer ` + `cloudy` = the icon `weer cloudy`, `weer default` when there is none) |
 | Progress bar | Filled from the left by a Home Assistant number: value x multiplier / "full at" (a percentage: full at 100). Width, height, with or without a frame. Optionally the percentage on top (e.g. 42%, rounded down, in the chosen font): its dots invert the bar, so it stays readable on the filled and the empty part; left out when it doesn't fit inside (Small needs a height of 7 with a frame) |
 | Game of Life | [Conway's Game of Life](https://conwaylife.com/wiki/Conway%27s_Game_of_Life) in an area (the whole display by default), one per page. Start: random dots (density %), one pattern, or the patterns whose names start with a text (e.g. `life_`) in turn or in random order, in the middle or at a random place. Edges: wrap around, dead, or a hidden margin (the world is 16 dots larger on every side, so gliders fly out of view and disappear). Speed in generations per second. It restarts when it stops changing (still, or repeating within 16 generations) and/or after a number of generations, after running on for a few seconds (an empty board at once) |
@@ -80,7 +80,11 @@ is shown instead. The display schedule ([Settings](#settings)) still decides whe
 
 ## Stopwatch
 
-Start, stop and reset the stopwatch that pages can show (also in Home Assistant).
+Start, stop and reset the stopwatch that pages can show (also in Home Assistant). It
+counts in real time from its start, so it keeps counting while the display is off or
+restarting (it is stored in NVS), and can run for years. Stop pauses it, Start continues.
+**Set start** makes it count from a date and time you pick (e.g. days since an event).
+Start and Set start need the synced time.
 
 ![Stopwatch](../Pictures/web-ui/stopwatch.png)