fix(cover): sanitize server_index_key so Windows :port URLs work (#889)

* fix(cover): sanitize server_index_key on disk so Windows accepts ":port" URLs

`serverIndexKeyFromUrl` (frontend) strips the URL scheme and leaves the rest of
the host as the index key — for a Navidrome instance running on the default
`:4533` port that is `host:4533/...`. On Linux/macOS the `:` is fine as a path
segment; on Windows `CreateDirectory` rejects the whole path with
`ERROR_INVALID_NAME` (os error 123). Result: every `cover_cache_ensure` and
`cover_cache_peek_batch` rejected its promise and the album / now-playing /
mainstage / lightbox surfaces stayed blank. Empirically verified — switching
the active server to a colon-free reverse-proxy URL made the covers load again
without any other change.

Centralize the fix in a new `cover_server_dir(root, key)` helper next to the
existing `cover_entity_relative_dir`: it runs `sanitize_path_segment` on the
server key the same way kind/entity ids are already cleaned, so `host:4533`
becomes `host_4533` and embedded URL paths collapse into one flat bucket
instead of nested directories. Every call site that wants the server bucket —
`cover_dir`, `count_cached_cover_ids`, `dir_usage_for_server`, the clear-server
command, and `clear_cover_fetch_failures` in the backfill worker — now goes
through it.

The on-disk layout changes (no more colons, no more nested URL paths), so bump
`LAYOUT_STAMP` to `canonical-segment-v4`. The existing stamp-mismatch sweep at
startup wipes the legacy buckets — users with a previously-working
(colon-free) layout rebuild the cache lazily as they browse. Library, offline,
and hot-cache data are not touched.

Adds two unit tests covering the sanitization on `cover_server_dir` and the
`cover_dir` passthrough.

Follow-up to #878 (which introduced `cover_cache_layout.rs` with
`sanitize_path_segment` applied only to kind/entity_id).

* chore(windows): silence dead_code warnings in debug taskbar_win build

`lib.rs` gates `taskbar_win::init` on `cfg(not(debug_assertions))` (PR #866 —
debug runs alongside an installed release instance and must not fight it for
the taskbar subclass). The `update_taskbar_icon` command still ships in debug
and early-returns until `init` populates the COM/HWND atomics, so the
init-only helpers (icon HICONs, button IDs, subclass plumbing, `make_buttons`,
`subclass_proc`, `init` itself) all look unused — 14 dead_code warnings on
every Windows debug `cargo build`.

File-level `#![cfg_attr(debug_assertions, allow(dead_code))]` suppresses those
warnings only in the debug profile. Release builds keep the strict dead-code
check, so a real removal would still surface there.

* docs(release): CHANGELOG for windows cover-cache server-key fix (PR #889)

src-tauri/src/cover_cache/mod.rs

@@ -10,7 +10,8 @@ use encode::write_webp_tier;
 use fetch::build_cover_art_url;
 use image::{DynamicImage, ImageReader};
 use psysonic_core::cover_cache_layout::{
-    count_entities_with_canonical_tier, cover_root_disk_usage, server_cover_disk_usage,
+    count_entities_with_canonical_tier, cover_root_disk_usage, cover_server_dir,
+    server_cover_disk_usage,
 };
 use psysonic_library::cover_backfill::{
     clear_cover_fetch_failures, collect_cover_backfill_batch, collect_cover_progress,
@@ -400,7 +401,7 @@ fn spawn_derive_remaining_tiers(
 
 /// Entity dirs with canonical `800.webp` under `album/` and `artist/` (segment layout).
 pub(crate) fn count_cached_cover_ids(root: &Path, server_index_key: &str) -> i64 {
-    let keyed = count_entities_with_canonical_tier(&root.join(server_index_key));
+    let keyed = count_entities_with_canonical_tier(&cover_server_dir(root, server_index_key));
     if keyed > 0 {
         return keyed;
     }
@@ -419,7 +420,7 @@ pub(crate) fn count_cached_cover_ids(root: &Path, server_index_key: &str) -> i64
 }
 
 pub(crate) fn dir_usage_for_server(root: &Path, server_index_key: &str) -> (u64, u64) {
-    server_cover_disk_usage(&root.join(server_index_key))
+    server_cover_disk_usage(&cover_server_dir(root, server_index_key))
 }
 
 pub(crate) fn dir_usage_at_root(root: &Path) -> (u64, u64) {
@@ -693,7 +694,7 @@ pub async fn cover_cache_clear_server(
 ) -> Result<(), String> {
     let st = state(&app)?;
     let guard = st.lock().await;
-    let path = guard.root.join(&server_index_key);
+    let path = cover_server_dir(&guard.root, &server_index_key);
     if path.is_dir() {
         std::fs::remove_dir_all(&path).map_err(|e| e.to_string())?;
     }