Fix expired trial plan selection flow

[Krisor1986]

Problem

When a free trial expired, protected pages only displayed an inactive-subscription warning and a retry button. The existing plan page could only register a new company, so the customer could not activate a plan while keeping the current company and data.

Result

• redirects expired authenticated companies to plan selection
• hides the free-trial option immediately for authenticated companies
• explains that the trial ended and cannot be requested again
• activates the selected plan on the existing company through an authenticated endpoint
• preserves the original trial dates as the record that the trial was already used
• limits plan activation to company owners and administrators
• returns the customer to the dashboard after activation
• keeps the new-account flow unchanged, including its first free trial

The payment remains the existing academic simulation; no real charge is made.

Validation

• backend suite: 203 passed, 2 skipped
• frontend tests: 11 passed
• npm run lint
• npm run build
• explicit regression coverage for expired-trial activation and second-trial rejection