feat: reduce long-running memory use and simplify desktop runtime

[rianjs]

What changed

• Park the large Last.fm importer payload while its window is closed, reload it on demand, and keep compact owner metadata resident. Remove the allocator-trim helper, which released zero bytes in the diagnostic run.
• Cache local cover art as raw files under a 512 MiB cap and reduce redundant copies across persistence, playback, and Spotify paths.
• Include the previously approved performance cleanup for playback rendering, playlist windowing, hidden presentation, startup loading, queue refreshes, plus related tests and documentation.

Measurements

On a private 245,941-scrobble fixture, parking took 35–42 ms and reload took 246–254 ms. The initial Rust test-process footprint was about 286 MB; parked samples ranged from about 55 MB to 145 MB across three cycles, with the third cycle showing no reduction. These are variable observations from the isolated test process, not the full app, WebView, or graphics memory, and do not establish a guaranteed saving.

Validation

• Rust workspace: 915 passed, 10 ignored; focused ownership and portability regressions passed. Formatting and workspace Clippy passed.
• Frontend: 177 tests passed, 6 skipped; lint and production build passed. Documentation, release-contract, and Tauri ACL checks passed.
• GitHub Actions passed on Windows x64/ARM64, Ubuntu amd64/arm64, and macOS arm64, including native release tests and bundle builds.
• The unique-ID macOS debug build compiled, but a locked Mac prevented interactive UI smoke.
• No new dependencies.