Injectable filesystem roots for core::paths to unit-test first-run flows

[LargeModGames]

Is your feature request related to a problem? Please describe.

The first-run selected-source path has no unit coverage and cannot safely get any today. Everything past the source picker in core/first_run.rs (apply_selections) persists immediately through the real config/state directories: ClientConfig::init_default_spotify_config(), UserConfig::save_config(), and core::state::save(default_state_path()?, ...) all resolve their locations through core::paths, which reads process env (XDG_*). Repo tests deliberately never mutate process env (one process, shared threads), so any test driving this path would write into the developer's actual config directory. Raised during review of #460.

Describe the solution you'd like

Injectable filesystem roots in core::paths (explicit root parameters or a test-only override), so config/state/cache locations can be pointed at a tempdir per test. On top of that:

• extend ScriptedOnboarding (core/test_helpers.rs) with configurable pick_sources output (it currently always returns None);
• add regression tests through run_first_run_picker for free-only and Spotify-plus-free selections, covering client.yml seeding, active-source persistence, and per-source setup.

Describe alternatives you've considered

• Setting XDG_* env vars inside tests: rejected, env is process-global and tests share threads.
• Extracting more of the flow into pure functions: helps, but the persistence half stays unverified without redirectable paths.

Additional context

Discussion: #460 (comment)

[spotatui]

🎧 spotatui triage

Auto-triaged with DeepSeek V4-Flash.

Label: enhancement
Likely area: core::paths and first-run test helpers

Summary

The issue requests injectable filesystem roots in core::paths so that first-run flows can be unit-tested without writing to the developer's real config directories. Currently the source picker path lacks unit coverage because tests would mutate process-global environment variables or touch real user config. The author proposes test-only overrides and additional regression tests for source selection flows.

Key points

• First-run selected-source path has no unit coverage and cannot safely get any today
• apply_selections in core/first_run.rs persists immediately via real config/state directories
• Locations resolved through core::paths which reads process env (XDG_*)
• Repo tests deliberately never mutate process env due to shared threads
• Proposed: injectable filesystem roots in core::paths, extend ScriptedOnboarding with configurable pick_sources output, add regression tests through run_first_run_picker

What would help

• No specific spotatui version mentioned
• No OS/platform stated
• No terminal environment details

Possible duplicates: none found

_{Compared against the 22 most recently updated open issues.}