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/config/config.go

@@ -41,6 +41,33 @@ type Config struct {
 	// active layout from its linked UniFi Protect live view (ProtectView).
 	// Requires controller credentials available to the daemon. 0 = off.
 	ViewRefreshSeconds int `yaml:"view_refresh_seconds,omitempty"`
+
+	// Clock overlays a live clock in a screen corner.
+	Clock Clock `yaml:"clock,omitempty"`
+}
+
+// Clock configures the on-screen clock overlay: a small always-on-top window
+// showing the current local time, drawn as outlined white text over the video
+// so it stays legible on both bright (day) and dark (night) scenes.
+type Clock struct {
+	// Enabled turns the overlay on.
+	Enabled bool `yaml:"enabled"`
+	// Timezone is an IANA name (e.g. "America/Denver"); "Local" or empty uses
+	// the system timezone. DST is handled automatically.
+	Timezone string `yaml:"timezone,omitempty"`
+	// Format is a Go time layout. Default "15:04:05" (24-hour with seconds).
+	// Examples: "3:04:05 PM", "Mon Jan 2  15:04".
+	Format string `yaml:"format,omitempty"`
+	// Corner places the overlay: bottom-right (default), bottom-left,
+	// top-right, top-left.
+	Corner string `yaml:"corner,omitempty"`
+	// FontSize is the glyph height in pixels (default 44).
+	FontSize int `yaml:"font_size,omitempty"`
+	// Width/Height are the overlay window size in pixels (defaults 300x72).
+	Width  int `yaml:"width,omitempty"`
+	Height int `yaml:"height,omitempty"`
+	// Margin is the gap from the screen edges in pixels (default 24).
+	Margin int `yaml:"margin,omitempty"`
 }
 
 // Controller holds UniFi Protect connection details.
@@ -306,6 +333,29 @@ func (c *Config) Defaults() {
 	if c.Player.RestartBackoffSeconds == 0 {
 		c.Player.RestartBackoffSeconds = 3
 	}
+	if c.Clock.Enabled {
+		if c.Clock.Timezone == "" {
+			c.Clock.Timezone = "Local"
+		}
+		if c.Clock.Format == "" {
+			c.Clock.Format = "15:04:05"
+		}
+		if c.Clock.Corner == "" {
+			c.Clock.Corner = "bottom-right"
+		}
+		if c.Clock.FontSize == 0 {
+			c.Clock.FontSize = 44
+		}
+		if c.Clock.Width == 0 {
+			c.Clock.Width = 300
+		}
+		if c.Clock.Height == 0 {
+			c.Clock.Height = 72
+		}
+		if c.Clock.Margin == 0 {
+			c.Clock.Margin = 24
+		}
+	}
 }
 
 // Validate checks referential integrity and returns the first problem found.
@@ -354,6 +404,14 @@ func (c *Config) Validate() error {
 	if c.ActiveLayout != "" && !layoutNames[c.ActiveLayout] {
 		return fmt.Errorf("active_layout %q is not a defined layout", c.ActiveLayout)
 	}
+
+	if c.Clock.Enabled && c.Clock.Corner != "" {
+		switch c.Clock.Corner {
+		case "bottom-right", "bottom-left", "top-right", "top-left":
+		default:
+			return fmt.Errorf("clock.corner %q must be one of bottom-right, bottom-left, top-right, top-left", c.Clock.Corner)
+		}
+	}
 	return nil
 }