[francisrath]

Draft, not ready to merge. This hasn't been tested enough yet: one developer, one Brother QL, the Docker dev setup. I'm opening it to get your feedback on the approach before polishing it.

Part of #1625. This is the smallest of three alternative PRs; each one builds on the previous:

1. This PR: a "Print" button that hands the label to a print agent on the user's computer. No database changes.
2. Print jobs for print agents (server-side queue) #1627: Part-DB keeps a job queue that agents fetch from. Printing then works from any device, phones included.
3. Instant print job delivery via Mercure (+ FrankenPHP Mercure 1.x fix) #1628: jobs reach the agent instantly through Mercure instead of at its next poll.

Pick whichever level you're comfortable with, or none.

What it does

• A new page under Tools → Print agent (/label/agent) connects the current browser to a print agent. The agent's settings page links there with #url=<agent>&key=<pairing key>. The fragment never reaches the server, and the connection is kept only in this browser's local storage.
• Once connected:
  • A Print button appears next to Download in the label generator, for saved profiles.
  • A printer icon appears next to each profile in the label menus of parts, part lots and storage locations.
• Clicking either opens a signed URL on the agent in a new window (/v1/handoff?src&type&ids&profile&ts&sig, HMAC-SHA256 with the pairing key). The agent then fetches the PDF itself through the existing POST /api/labels/generate with its own read-only API token.

This is a top-level navigation, so it needs no CSP change and isn't affected by Chrome's local-network fetch rules. Part-DB never talks to the agent itself and stores nothing on the server.

The agent contract is documented in docs/usage/labels.md ("Printing with a print agent"). It isn't tied to one agent: any program that implements the handoff URL works. I wrote one for Brother QL printers, pdb-print (MIT).

Limitations

• Only saved profiles can be printed; unsaved changes in the label generator are not printed.
• If the browser blocks the popup, the Part-DB tab itself navigates to the agent.

Tests

• tests/Controller/LabelControllerTest.php: the connect page, and the buttons with and without a profile.
• /label/agent is added to the availability test.
• The JS helper (id normalisation, signing) was only tested with throwaway node tests, which are not part of this PR.
• Tested by hand: connecting the agent, and printing from the label dialog and the menu icons (Chrome, macOS, against the Docker dev setup).

Screenshots

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