Add TOML dependency and new config/startup modules

The TOML dependency is added to `Cargo.lock` to support configuration
file parsing.
New modules `config` and `startup` are introduced in
`doctate-client-core`:
- `config`: Handles client configuration loading and saving, including
  defaults for polling intervals and window hours.
- `startup`: Contains routines for initial cleanup tasks, such as
  removing stale markers and orphan audio files based on defined
  retention periods.

doctate-common/src/timestamp.rs

@@ -1,5 +1,5 @@
-use time::OffsetDateTime;
 use time::format_description::well_known::Rfc3339;
+use time::{OffsetDateTime, UtcOffset};
 
 /// Current UTC time as an RFC3339 string with second precision, e.g.
 /// `2026-04-15T10:32:00Z`. Subsecond fractions are deliberately stripped
@@ -36,6 +36,27 @@ pub fn filename_stem_to_recorded_at(stem: &str) -> String {
     }
 }
 
+/// Parse an RFC3339 timestamp. Returns `None` on any error so callers
+/// can fall back to a lenient display path without having to classify
+/// failures.
+pub fn parse_rfc3339(s: &str) -> Option<OffsetDateTime> {
+    OffsetDateTime::parse(s, &Rfc3339).ok()
+}
+
+/// Extract `HH:MM` in the local timezone from an RFC3339 UTC timestamp.
+/// On parse failure, returns the raw string truncated to its first 16
+/// characters — the user still sees something readable even if the
+/// value is shaped unexpectedly.
+pub fn extract_hhmm(ts: &str) -> String {
+    match parse_rfc3339(ts) {
+        Some(t) => {
+            let local = t.to_offset(UtcOffset::current_local_offset().unwrap_or(UtcOffset::UTC));
+            format!("{:02}:{:02}", local.hour(), local.minute())
+        }
+        None => ts.chars().take(16).collect(),
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -88,4 +109,35 @@ mod tests {
         );
         assert!(now.ends_with('Z'), "expected UTC 'Z' suffix, got {now}");
     }
+
+    #[test]
+    fn parse_rfc3339_accepts_utc_z() {
+        let t = parse_rfc3339("2026-04-18T10:32:00Z").expect("parse UTC");
+        assert_eq!(t.hour(), 10);
+        assert_eq!(t.minute(), 32);
+    }
+
+    #[test]
+    fn parse_rfc3339_returns_none_on_garbage() {
+        assert!(parse_rfc3339("not a timestamp").is_none());
+    }
+
+    /// `extract_hhmm` depends on the host timezone for formatting, so we
+    /// can't assert an exact value — but we can assert the shape and
+    /// that the fallback kicks in on unparseable input.
+    #[test]
+    fn extract_hhmm_shape_is_two_colon_two() {
+        let s = extract_hhmm("2026-04-18T10:32:00Z");
+        assert_eq!(s.len(), 5, "expected HH:MM, got {s}");
+        assert!(s.chars().nth(2) == Some(':'), "expected colon at index 2: {s}");
+    }
+
+    #[test]
+    fn extract_hhmm_falls_back_to_truncated_raw_on_parse_failure() {
+        assert_eq!(extract_hhmm("not-a-timestamp"), "not-a-timestamp");
+        assert_eq!(
+            extract_hhmm("way-longer-than-sixteen-chars"),
+            "way-longer-than-"
+        );
+    }
 }