Fix default-channel toggle in panel settings and protect the default channel

[glennjacobs]

Summary

QA of the panel's Settings → Channels section surfaced one bug and one gap:

• Bug — the default toggle did nothing. The shared Toggle component was written against radix-vue's old Switch API (:checked / @update:checked); reka-ui v2 renamed these to modelValue / update:modelValue, so the switch never emitted and default could never be changed from the create dialog or edit page. Fixed the binding (public on/toggle API unchanged) and added a component test so API drift fails loudly.
• Gap — the default channel wasn't actually protected. The channels table hid the delete row action for the default channel, but the edit page and the core actions didn't enforce anything: the default channel could be deleted from its edit screen, and its default flag toggled off, leaving the store with no default channel.

Default-channel protection

• DeleteChannel rejects the default channel (alongside the existing order-history guard).
• UpdateChannel rejects unsetting default on the current default — the flag moves by promoting another channel, so a store with channels always has a default.
• The panel edit page disables the delete button and locks the default toggle for the default channel, with explanatory tooltip/hint (Toggle gains a disabled prop); the controller maps both failures to translated flash messages as a backstop.
• New strings translated in all 16 locales; spec 0049's channels section updated to record the rule.

Note: the legacy Filament admin doesn't route channel deletes through these core actions, so it can still delete a default channel — wiring its ChannelResource through DeletesChannel/UpdatesChannel would be a sensible follow-up.

Testing

• Core: new unit tests for both guards; suite 949 passed.
• Panel: new feature tests asserting the flash errors and unchanged records; suite 236 passed.
• Frontend: new Toggle tests (emit on click, no emit when disabled, state rendering); vitest 38 passed, type-check and build clean.
• PHPStan and Pint clean.

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