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)

CHANGELOG.md

@@ -485,6 +485,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 
 
+### Covers — load on Windows when the server URL has a `:port`
+
+**By [@Psychotoxical](https://github.com/Psychotoxical), PR [#889](https://github.com/Psychotoxical/psysonic/pull/889)**
+
+* Album, now-playing, mainstage, and lightbox covers no longer stay blank on Windows when the active server URL has a `:port` (typical Navidrome LAN setup on `:4533`). The colon used to land in a Windows filesystem segment, so the OS rejected the whole cache path with `ERROR_INVALID_NAME` and every cover load failed silently.
+* Existing cache buckets on disk are wiped once on the next launch (layout-stamp bump) and rebuild lazily as users browse. Library, offline, and hot caches are untouched.
+
+
+
 ## [1.46.0] - 2026-05-18
 
 > **🙏 Special thanks to [@zz5zz](https://github.com/zz5zz)** for his tireless quirk-spotting and bug reports on the [Psysonic Discord](https://discord.gg/AMnDRErm4u) — several of the polish fixes in this release landed directly off the back of his messages.