Bug fixes:
- Fix tt report showing wrong end date and negative duration for running tasks
  by treating zero EndAt as time.Now() (markdown.go)
- Fix panic when pressing r on an empty task list (main.go)
- Fix empty task being created on blank Enter keypress (main.go)
- Fix temp file accumulation in tt edit by adding defer os.Remove (edit.go)
- Fix file descriptor leak in tt from-json by adding defer tmp.Close (fromjson.go)

Code quality:
- Change model.Bytes() to return ([]byte, error) instead of log.Fatalln on error
- Extract setTask helper in store.go to eliminate triplicated serialize-and-store pattern
- Extract effectiveEndAt helper in project_timer.go; capture time.Now() once per call
- Remove debug log.Println calls throughout the TUI Update loop
- Fix typo: newRerportCmd -> newReportCmd
- Replace deprecated ioutil.TempFile with os.CreateTemp
- Restrict exported JSON file permissions from 0o666 to 0o644
- Improve EDITOR missing error message to be platform-neutral

…ilters, list stats

New CLI commands:
- tt start <title>: stop any running task and immediately start a new one
- tt stop: stop all running or paused tasks and print what was stopped
- tt pause: freeze the current task's timer (time stops accumulating)
- tt resume: continue a paused task from where it left off

True pause/resume (same task entry, not a new one) required a data model
extension: Task gains PausedAt and PausedFor fields (backwards-compatible
via omitempty). CloseTasks now finalizes paused tasks at their pause time
so paused-dead time is never counted.

TUI changes:
- p key: toggle pause/resume on the active task; paused tasks show [paused]
- d key: delete the selected task (no confirmation, consistent with tool ethos)
- Item description now uses taskDuration which is pause-aware

Report improvements:
- --since and --until flags (YYYY-MM-DD) filter tasks by start date
- Total time in the report header reflects the filtered set only
- Paused tasks render as "paused" instead of "in progress" in the end column

List improvements:
- -v / --verbose flag shows task count and total time per project