Add on-screen clock overlay (outlined text, configurable corner/timezone)

A small always-on-top mpv window renders the local time in a screen
corner. Implementation notes:

- Transparent lavfi canvas (color=...@0.0, --alpha=yes) so the camera
  video shows through; no new dependencies. If the compositor can't do
  alpha it degrades to a dark backing, still legible.
- Time drawn as an ASS osd-overlay pushed over mpv IPC once a second:
  white fill + black outline (\bord), so it reads on both bright (day)
  and dark (night) scenes without sampling the picture. Formatting the
  text in Go avoids any filtergraph escaping.
- Time computed with time.LoadLocation against a configured IANA zone
  (default "Local"), so it's correct regardless of the host clock's zone
  and handles DST. A bad zone name fails at startup.
- Managed on the daemon's own context (survives layout switches); the
  daemon keeps it positioned and raised above camera tiles. ensureClock
  is a no-op when the clock config + resolution are unchanged, so a
  reload never disturbs it.

Config: new `clock` section (enabled, timezone, format, corner,
font_size, width, height, margin) with defaults and corner validation.
Documented in README and config.example (shipped enabled, America/Denver,
24-hour w/ seconds). Tests cover corner geometry.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -155,6 +155,27 @@ wall — stream URLs, tile geometry, player settings — against what's already
 running and leaves the streams untouched when nothing material changed, so
 saving an unrelated edit never blanks the screen.
 
+## Clock overlay
+
+An optional always-on-top clock can be rendered in a screen corner:
+
+```yaml
+clock:
+  enabled: true
+  timezone: America/Denver     # IANA name; "Local" uses the system zone. DST auto.
+  format: "15:04:05"           # Go time layout (24-hour w/ seconds)
+  corner: bottom-right         # bottom-right | bottom-left | top-right | top-left
+```
+
+It's a tiny transparent mpv window (no extra dependencies) drawing the time as
+**white text with a black outline**, so it stays readable over both bright (day)
+and dark (night) camera scenes without measuring the picture. The time is
+formatted in the configured timezone via Go's zone database, so it's correct
+regardless of the host clock's zone and handles DST on its own. The daemon keeps
+it positioned and on top across layout switches. Size (`width`/`height`),
+`font_size`, and edge `margin` are configurable; enable/disable or retune it
+live with an edit plus `rtsp-streamer reload`.
+
 ## UniFi Protect live views
 
 Copy a saved Protect "Live View" (its cameras and grid) straight into a layout: