[breadcrumbs] Task 5: Breadcrumbs — Mode enum, constructors, mode switching

[web-padawan]

Implementation task 5 of the Breadcrumbs Flow component (spec-driven development pipeline).

What to do:

• Add the public nested Mode enum (ROUTER, MANUAL) and the two constructors: Breadcrumbs() (defaults to Mode.ROUTER) and Breadcrumbs(Mode)
• Implement getMode() and setMode(Mode) with the symmetric clear-and-rewire contract: both transitions discard existing children; ROUTER → MANUAL also unregisters the navigation listener (once it exists, Task 7); MANUAL → ROUTER triggers an initial trail build
• setMode with the current mode value is a no-op
• The Mode.ROUTER guard on add/remove/removeAll is NOT added here — it lands in Task 6 (the inherited child-management methods are already available from Task 1). Router listener wiring lands in Task 7; here setMode(ROUTER) only flips the internal flag and clears children.

Spec references:

• flow-tasks.md — Task 5
• flow-spec.md — Key Design Decisions §1 + §13, Component Classes → Breadcrumbs

Depends on #9377 (Task 1).

Part of vaadin/web-components#7960

[vaadin-bot]

This ticket/PR has been released with Vaadin 25.2.0-rc1.