Add --weekly flag to csv command

- Add `write_csv_weekly()` to output.py: writes entries from multiple
  days as a single CSV with one header row, correct date per row
- Add `-w`/`--weekly` flag to csv subparser
- _cmd_csv branches on args.weekly: fetches week sections, formats
  per-day date strings, calls write_csv_weekly; --raw is honoured
- Add TestWriteCsvWeekly with 6 tests
- Update README with weekly csv usage examples

src/timesheets/output.py

@@ -31,6 +31,32 @@ def to_csv_entries(rows: list[dict]) -> list[dict]:
     ]
 
 
+def write_csv_weekly(
+    day_sections: list[tuple[str, list[dict]]],
+    output: IO[str],
+    project_map: dict,
+) -> None:
+    """Write entries from multiple days as a single CSV with one header row.
+
+    day_sections is a list of (date_str, entries) pairs where entries are
+    already write_csv-compatible (project, description, quantity).
+    """
+    writer = csv.writer(output)
+    writer.writerow(["Date*", "Project*", "Task", "Description", "Quantity"])
+    for date_str, entries in day_sections:
+        for entry in entries:
+            project, task = resolve_project_task(entry["project"], project_map)
+            writer.writerow(
+                [
+                    date_str,
+                    project,
+                    task,
+                    entry["description"],
+                    f"{entry['quantity']:.2f}",
+                ]
+            )
+
+
 def write_csv(
     aggregated: list[dict],
     output: IO[str],