Rename dashboard widget key active_timers → timers and add migration

[multiplex55]

Motivation

• Standardize the timers widget key to timers and update default dashboard layout to the new name.
• Ensure existing user configs that still reference the legacy active_timers key continue to load without data loss.

Description

• Register the timers widget under the new name by changing the entry in WidgetRegistry::with_defaults() from "active_timers" to "timers" in src/dashboard/widgets/mod.rs.
• Change the default dashboard slot in DashboardConfig::default() to use "timers" in src/dashboard/config.rs.
• Add migrate_active_timers_widgets() and invoke it from DashboardConfig::sanitize() to detect slot.widget == "active_timers", rewrite it to "timers", merge the default settings via merge_json, and append a migration warning string.

Testing

• No automated tests were executed as part of this change.
• The code compiles locally as part of development (manual verification), but cargo test was not run in this rollout.

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Codex Task