Add stopwatch dashboard widget

[multiplex55]

Motivation

• Provide an on-dashboard view for stopwatches so active and paused timers can be monitored with names and elapsed time shown.
• Expose in-dashboard controls to Pause, Resume, and Stop stopwatches to match the plugin's existing actions.
• Keep the dashboard widget behaviour consistent with other widgets by supporting refresh intervals and manual refresh.

Description

• Added src/dashboard/widgets/stopwatch.rs implementing StopwatchWidget with StopwatchConfig, a TimedCache of StopwatchRows, refresh logic, and a header_ui refresh button.
• The widget queries stopwatches via crate::plugins::stopwatch::all_stopwatches() and formats elapsed time with crate::plugins::stopwatch::format_elapsed(id) for display.
• Each entry renders buttons that produce WidgetActions with actions stopwatch:pause:<id>, stopwatch:resume:<id>, and stopwatch:stop:<id>, and sets appropriate query_overrides.
• Registered and exported the widget by adding StopwatchWidget to WidgetRegistry::with_defaults and the module exports in src/dashboard/widgets/mod.rs.

Testing

• No automated tests were executed for this change.

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Codex Task