feat(input): implement multi-seat support and virtual device naming

[willybarret]

Description

This change introduces multiseat input support and virtual device naming on Linux, allowing multiple users to interact with Sunshine independently on the same system. The intention is to provide flexibility for shared workstations, gaming setups, and other advanced multi-user environments.

Modern systems (based on systemd and logind) have the concept of a seat (seat0 is default), with associated input devices and graphics outputs. For example, look at loginctl seat-status seat0.
When Sunshine fires up uinput, it’s logically similar to plugging in a second USB mouse: the input gets attached to the default seat (seat0).
While I don't know exactly what the solution should look like, (it) should ideally run as a separate "seat" (as far as logind and libinput are concerned).

from #137 (comment)

Screenshot

Issues Fixed or Closed

Roadmap Issues

Type of Change

• feat: New feature (non-breaking change which adds functionality)
• fix: Bug fix (non-breaking change which fixes an issue)
• docs: Documentation only changes
• style: Changes that do not affect the meaning of the code (white-space, formatting, missing semicolons, etc.)
• refactor: Code change that neither fixes a bug nor adds a feature
• perf: Code change that improves performance
• test: Adding missing tests or correcting existing tests
• build: Changes that affect the build system or external dependencies
• ci: Changes to CI configuration files and scripts
• chore: Other changes that don't modify src or test files
• revert: Reverts a previous commit
• BREAKING CHANGE: Introduces a breaking change (can be combined with any type above)

Checklist

• Code follows the style guidelines of this project
• Code has been self-reviewed
• Code has been commented, particularly in hard-to-understand areas
• Code docstring/documentation-blocks for new or existing methods/components have been added or updated
• Unit tests have been added or updated for any new or modified functionality

AI Usage

• None: No AI tools were used in creating this PR
• Light: AI provided minor assistance (formatting, simple suggestions)
• Moderate: AI helped with code generation or debugging specific parts
• Heavy: AI generated most or all of the code changes

[sonarqubecloud]

Quality Gate failed

Failed conditions
1 New issue
1 New Code Smells (required ≤ 0)

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