feat: show build date on the display-manager interface

Add a build.rs that stamps CCE_BUILD_DATE at compile time and render it in the
bottom-right corner, so it's clear which build the greeter is running.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

 build.rs    | 16 ++++++++++++++++
 src/main.rs | 14 ++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/build.rs b/build.rs
new file mode 100644
index 0000000..1b685a6
--- /dev/null
+++ b/build.rs
@@ -0,0 +1,16 @@
+use std::process::Command;
+
+fn main() {
+    // Capture the build date so the interface can show which build is running.
+    // With no rerun-if-changed pin, Cargo re-runs this whenever the crate's
+    // sources change, so CCE_BUILD_DATE tracks the last actual recompile.
+    let date = Command::new("date")
+        .arg("+%Y-%m-%d")
+        .output()
+        .ok()
+        .and_then(|o| String::from_utf8(o.stdout).ok())
+        .map(|s| s.trim().to_string())
+        .filter(|s| !s.is_empty())
+        .unwrap_or_else(|| "unknown".to_string());
+    println!("cargo:rustc-env=CCE_BUILD_DATE={}", date);
+}
diff --git a/src/main.rs b/src/main.rs
index 12de5b3..d9cd20a 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -441,6 +441,20 @@ impl State {
             bounds: None,
         });
 
+        // Build-date stamp in the bottom-right corner.
+        let build_buf = make_text_buffer(
+            &mut self.font_system,
+            concat!("Built ", env!("CCE_BUILD_DATE")),
+            11.0,
+        );
+        self.text_items.push(TextItem {
+            buffer: build_buf,
+            x: self.width - 130.0,
+            y: self.height - 24.0,
+            color: glyphon::Color::rgb(0x60, 0x60, 0x6e),
+            bounds: None,
+        });
+
         let mut widget_labels = Vec::new();
         for w in self.widgets_iter() {
             widget_labels.extend(w.text_labels());