feat(timesheet): add configuration model with period validation

Add TimesheetConfig and Period structs for configuring timesheet
periods with expected working hours per week. Include RepositoryConfig
for loading .streamd.toml with timezone and timesheet configuration.

Validation ensures:
- No overlapping periods
- Valid date ranges (start <= end)

Also adds chrono-tz dependency for timezone support.

Cargo.toml

@@ -18,6 +18,7 @@ pulldown-cmark = "0.13"
 regex = "1"
 once_cell = "1"
 chrono = { version = "0.4", features = ["serde"] }
+chrono-tz = "0.9"
 walkdir = "2"
 indexmap = { version = "2", features = ["serde"] }
 itertools = "0.14"