feat: copy QuickConnect code to clipboard on tap

[v3DJG6GL]

Pull Request Description

Tapping the QuickConnect code copies it to the clipboard. This makes logging in much more convenient, especially when logging into Fladder on a device where a Jellyfin client is already set up :)

• This feature was part of PR feat: Seerr QC login, password-login hide, auto-URL scheme, new ENV vars | fix: Set-Cookie #812

Testing Build

ghcr.io/v3djg6gl/fladder:test-qc-copy-to-clipboard

Issue Being Fixed

• Addresses [Feature Request] QuickConnect: copy code to clipboard with a tap/click #653

Screenshots / Recordings

N/A

Notes

AI Disclosure

Developed with assistance from Claude Code (Opus 4.6). The AI helped me identify the related files, code parts and proposed possible code modifications and fixes.
I've manually reviewed, cleaned, refactored and hardened the code the best I could, but since my Dart capabilities are not that good, I see this PR more as a proposal/idea.
A Proper code review is required in any case...

Checklist

• If a new package was added, did you ensure it works for all supported platforms? Is the package well maintained
• Check that any changes are related to the issue at hand.