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>

config.example.yaml

@@ -84,6 +84,21 @@ layouts:
 
 active_layout: quad
 
+# On-screen clock overlay: a small always-on-top window showing the local time,
+# drawn as white text with a black outline so it stays legible over both bright
+# (day) and dark (night) camera scenes. The time is computed in the given
+# timezone (DST handled automatically), independent of the host clock's zone.
+clock:
+  enabled: true
+  timezone: America/Denver     # IANA name; "Local" uses the system zone
+  format: "15:04:05"           # Go time layout (24-hour w/ seconds). Others:
+                               #   "3:04:05 PM"  ·  "Mon Jan 2  15:04"
+  corner: bottom-right         # bottom-right | bottom-left | top-right | top-left
+  font_size: 44                # glyph height in px
+  width: 300                   # overlay window size in px
+  height: 72
+  margin: 24                   # gap from the screen edges in px
+
 # Re-sync the active layout from its linked UniFi Protect live view every N
 # seconds (0 = off). Layouts created by `views import` carry a `protect_view:`
 # and are re-synced when this is set. Requires the controller password at