chore(deps): promote chrono to a direct dependency

Needed for DST-aware recurrence expansion (calendar/mod.rs); already
resolved transitively via sqlx, so this adds no new compiled crate.

src-tauri/Cargo.lock

@@ -5980,6 +5980,7 @@ dependencies = [
  "argon2",
  "async-trait",
  "chacha20poly1305",
+ "chrono",
  "docx-rs",
  "futures-util",
  "getrandom 0.2.17",

src-tauri/Cargo.toml

@@ -27,6 +27,9 @@ tokio = { version = "1", features = ["rt-multi-thread", "macros", "sync", "time"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 uuid = { version = "1", features = ["v4"] }
+# Local<->UTC, DST-aware — needed for calendar recurrence (T6.2); already in
+# the dependency tree transitively (sqlx), this just promotes it to direct.
+chrono = { version = "0.4", default-features = false, features = ["clock"] }
 
 # storage
 sqlx = { version = "0.8", features = ["runtime-tokio", "sqlite", "migrate"] }