Fix expired trial payment flow

[Krisor1986]

Problem

After an expired trial, selecting a new plan activated the subscription immediately and sent the user directly to the dashboard. This skipped the payment step.

Result

• Selecting a plan now opens a dedicated checkout page.
• The checkout shows the selected plan and monthly price.
• Subscription activation happens only after the simulated payment is confirmed.
• Users can return to plan selection before confirming.
• Only company owners and administrators can confirm the subscription.
• The page is available in English, Spanish and Portuguese.
• Test card values remain in the browser and are never sent or stored.

Payment remains an academic simulation and does not make a real charge.

Validation

• Frontend tests: 11 passed
• Subscription backend tests: 9 passed
• ESLint: passed
• Production build: passed
• Patch formatting: passed