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/daemon/clock_test.go

@@ -0,0 +1,29 @@
+package daemon
+
+import (
+	"testing"
+
+	"github.com/lwoodard/rtsp-streamer/internal/config"
+)
+
+func TestClockRectCorners(t *testing.T) {
+	cl := config.Clock{Width: 300, Height: 72, Margin: 24}
+	const w, h = 1920, 1080
+	cases := map[string][2]int{
+		"bottom-right": {1920 - 300 - 24, 1080 - 72 - 24},
+		"bottom-left":  {24, 1080 - 72 - 24},
+		"top-right":    {1920 - 300 - 24, 24},
+		"top-left":     {24, 24},
+		"":             {1920 - 300 - 24, 1080 - 72 - 24}, // default = bottom-right
+	}
+	for corner, want := range cases {
+		cl.Corner = corner
+		r := clockRect(w, h, cl)
+		if r.X != want[0] || r.Y != want[1] {
+			t.Errorf("corner %q: got (%d,%d), want (%d,%d)", corner, r.X, r.Y, want[0], want[1])
+		}
+		if r.W != 300 || r.H != 72 {
+			t.Errorf("corner %q: size (%d,%d), want (300,72)", corner, r.W, r.H)
+		}
+	}
+}