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>

internal/player/player.go

@@ -245,7 +245,14 @@ func (p *Player) Stop() {
 // Command sends a JSON IPC command to mpv and returns the decoded reply. Used
 // for health probes and live property changes.
 func (p *Player) Command(args ...any) (map[string]any, error) {
-	conn, err := net.DialTimeout("unix", p.ipcPath, 2*time.Second)
+	return ipcCommand(p.ipcPath, args...)
+}
+
+// ipcCommand sends one JSON command to an mpv IPC socket and returns the
+// decoded reply. Shared by Player (health probes, reload) and Clock (overlay
+// updates).
+func ipcCommand(path string, args ...any) (map[string]any, error) {
+	conn, err := net.DialTimeout("unix", path, 2*time.Second)
 	if err != nil {
 		return nil, err
 	}