Add SSH key pair management to Settings dashboard

[paulotruta]

Summary

Adds a new SSH Key Access section to the Settings page so users can manage direct SSH access to their Edgebox instance from the dashboard.

What's included

• TaskFactory — GENERATE_SSH_KEY, GET_SSH_KEY, REVOKE_SSH_KEY constants and factory methods.
• SettingsController — new actions wired into ALLOWED_ACTIONS/ACTION_CONTROLLER_TITLES, SSH key options read in index(), and a downloadPrivateKey route that serves the private key as a downloadable id_ed25519 attachment.
• HomeController — action descriptions and icons for the three SSH task types so they render properly in the Actions overview on the home page.
• settings/index.html.twig — SSH section with:
  • Step 1: Download private key button
  • Step 2: macOS/Linux and Windows connection instructions (tabbed)
  • Advanced panel: public key, fingerprint, connection command, regenerate/revoke buttons
  • Proper alert styling when no key exists
• settings/action.html.twig — status messages for generate_ssh_key, revoke_ssh_key, and get_ssh_key actions.

Why

Direct SSH access complements the existing web shell and browser dev environment by giving advanced users a persistent, scriptable terminal session. This PR is the dashboard half — the edgeboxctl half is edgebox-iot/edgeboxctl#44.

Test plan

• Settings page renders the SSH section for both the "no key" and "active key" states.
• Generate SSH Key button creates a task and redirects to the action page with a descriptive status.
• Download Private Key triggers a file download of id_ed25519.
• Regenerate and Revoke buttons show confirmation dialogs and dispatch the correct tasks.
• macOS/Linux and Windows tabs switch instructions correctly.
• Actions overview on the home page shows SSH tasks with proper descriptions and icons.
• Twig and PHP syntax validation passes.