fix(cover): restore full-resolution album and artist covers (#1205)

* fix(cover): build cover tiers from the full-resolution source

Derive the larger tiers from the decoded download instead of re-reading the just-written smaller tier (resize never upscales, so 512/800 were stored at the small resolution). Resolve the full-res 2000 tier exactly so it is actually downloaded and cached. Bump the cache layout stamp to drop already-poisoned tiers.

* fix(cover): open the cover lightbox at full resolution

Require the exact requested tier for the full-res helper so a smaller warmed tier no longer pins the lightbox to a downscaled image. Race the 2000 fetch against a 500ms opening window, show the 800 tier meanwhile, and persist 2000 for the next open. Adds an opening animation (reduced-motion aware) and tests.

* docs(changelog): note full-resolution cover fix (#1205)

* fix(cover): keep full-res peek exact in peek_batch and the grid seeder

Review follow-up: ensure-path peek alone left a hole — cover_cache_peek_batch still laddered a 2000 request down to a smaller tier, and the in-memory grid seeder wrote that smaller path under the 2000 key, so Hero/fullscreen/lightbox surfaces (which peek 2000 before ensure) still showed a downscaled cover. Share one exact-2000 rule (peek_plain_cover_tier) across ensure and peek_batch, and never seed the full-res key from a smaller tier file.

src-tauri/crates/psysonic-core/src/cover_cache_layout.rs

@@ -17,7 +17,7 @@
 use std::path::{Path, PathBuf};
 
 /// Written to `{cover_root}/.storage-layout` — mismatch triggers cache reset.
-pub const LAYOUT_STAMP: &str = "canonical-segment-v4";
+pub const LAYOUT_STAMP: &str = "canonical-segment-v5";
 
 /// True for ids that are only valid as `getCoverArt` targets, not library entity keys.
 pub fn is_fetch_only_cover_id(id: &str) -> bool {