️🛠 - Feature-flag desktop-only Rust code for mobile compilation

[maxwellmattryan]

Description

Prepare the Rust backend to compile for mobile targets (aarch64-apple-ios, aarch64-linux-android) by gating desktop-only code behind feature flags.

Scope:

• Add desktop and mobile feature flags to src-tauri/Cargo.toml
• Gate desktop-only services behind #[cfg(feature = "desktop")]:
  • services/device.rs — USB detection (sysinfo, diskutil)
  • services/export/ — Pioneer PDB/ANLZ generation
  • services/sync.rs — USB device sync
  • services/audio/ — rodio playback
  • services/media_controls.rs — souvlaki
  • services/analysis.rs — audio analysis (symphonia + stratum-dsp)
  • services/library.rs — file import/metadata extraction (lofty)
• Gate desktop-only dependencies:
  • rodio, souvlaki, raw-window-handle, sysinfo → desktop feature only
  • lofty, walkdir, stratum-dsp, binrw, flate2, base85 → desktop feature only
• Gate desktop-only commands in commands/ that reference gated services
• Gate desktop-only code in lib.rs (service init, command registration, menu)
• Ensure cargo check --target aarch64-apple-ios --no-default-features --features mobile passes
• Keep desktop as a default feature so existing builds are unaffected

Shared services that must compile for both targets:

• services/discovery/ — metadata fetching, stream resolution, all sub-modules
• services/playlist.rs — playlist CRUD
• services/tag.rs — tag/category CRUD
• services/settings.rs — app settings
• services/artwork.rs — artwork caching
• services/backup.rs — database backup
• proxy.rs — localhost audio proxy (needed for mobile preview playback)

Note: n_transform.rs (YouTube de-throttling) depends on Webview::eval() — this is architecturally shared and should compile for mobile, but needs runtime validation on iOS (see #80).

Companion issue: #81 (Mobile CI target) should be set up once this issue is complete to prevent regressions.

Key files:

• src-tauri/Cargo.toml
• src-tauri/src/lib.rs
• src-tauri/src/commands/*.rs
• src-tauri/src/services/device.rs, audio/, media_controls.rs, export/, sync.rs, analysis.rs, library.rs

Open questions

• Should walkdir be shared or desktop-only? (Used for file import traversal — desktop-only)
• Are there any implicit desktop dependencies in shared services (e.g., does artwork.rs use any desktop-only crate)?

Acceptance criteria

• Desktop build unchanged (cargo build works as before with default features)
• Mobile target compiles without desktop dependencies: cargo check --target aarch64-apple-ios --no-default-features --features mobile
• All shared services (discovery, tag, playlist, settings, artwork, backup) compile for both targets
• proxy.rs compiles for mobile target
• No runtime desktop assumptions in shared code paths

Tasks

TBD