fix: keep banned participants visible after reload, auto-rejoin on unban

[Phai2o]

Closes #4738.

• Participants.tsx: include banned participants in the list regardless of the connected/online filter, so a moderator can still reach the unban button after reloading the page (banned sessions are persisted as connected=false, which previously caused them to disappear).
• RejectionPage.tsx: when a user lands here with status="banned" and a boardId in the URL, silently probe the join endpoint every 30 seconds and surface a "Try to rejoin" button. On a successful re-admission the page dispatches permittedBoardAccess and the board mounts without a manual refresh. Probing goes through API.joinBoard directly instead of the joinBoard thunk so a still-banned response does not flip the redux status to pending and unmount the rejection page.
• i18n (en/de/fr): added bannedRetryHint, retry, retrying.
• styles: button group + retry button styling, dark mode respected.

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• I have performed a self-review of my own code
• I have written and understand every part of this contribution myself - if AI tools were used, I have thoroughly reviewed and verified all changes
• I have commented my code, particularly in hard-to-understand areas
• The light- and dark-theme are both supported and tested
• The design was implemented and is responsive for all devices and screen sizes
• The application was tested in the most commonly used browsers (e.g. Chrome, Firefox, Safari)

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[Schwehn42]

hey @Phai2o, thanks for the PR! on the first look, this looks pretty great. please make sure to update the snapshots by running

yarn run test --watch=false --update

so that the pipeline is happy:)