feat(ssh): display compact terminal QR code for web authentication URLs

[mtthsbrd]

• Add qrcode crate to workspace dependencies
• Implement render_qr_code_terminal in warpgate-common using ANSI background styling and Unicode lower half-blocks to eliminate line-height gaps
• Add database migration m00089_ssh_show_qr_code adding ssh_show_qr_code to the parameters table (defaults to true)
• Expose ssh_show_qr_code in Parameters entity and Admin API
• Add switch toggle under SSH section in Admin Web UI
• Update warpgate-protocol-ssh to read ssh_show_qr_code from Parameters and render QR code alongside web authentication instructions
• Add unit tests for terminal QR code rendering and SSH instruction formatting

Description

Sometimes the screen/computer displaying the URL for web authentication will not have a browser / copy&paste / whateverthereason. In these cases having a QR code displayed can improve UX: 1.) Scan the barcode, 2.) Authenticate on the device you scanned with 3.) Continue on terminal.

Per maintainer request, this is implemented as a dynamic parameter (ssh_show_qr_code) with a UI toggle in the Admin console rather than a static config file option.

AI Usage

Choose the level of AI involvement for this PR.

• Fully vibe coded
• AI-designed, AI-coded, manually checked
• Human-designed, AI-coded
• Human-designed, human-coded (includes AI autocompletions and boilerplate gen)

_{This is not to block AI contributions but rather to speed up PR review (saves time on trying to deduce the logic behind AI hallucinations).}

[Eugeny]

Thank you for the PR! Please update the option to be a Parameter fields with a UI toggle instead of a config file option.

[mtthsbrd]

Updated as requested!

• Rebased onto latest main.
• Added migration m00089_ssh_show_qr_code adding ssh_show_qr_code (boolean, default true) to the parameters table.
• Exposed ssh_show_qr_code in Parameters entity and Admin API.
• Added a switch toggle under the SSH section in the Web Admin UI (Terminal display -> Display QR code for login URLs).
• Updated openapi-schema.json.
• Updated warpgate-protocol-ssh to read ssh_show_qr_code from the database parameters instead of warpgate.yaml.
• Removed show_qr_code from SshConfig.