feat(themes): Theme Store install counts, downloads & sort (#1036)

* feat(themes): install counts, downloads, popularity & sort in the Theme Store

Each registry theme now carries an install count and a last-changed date. Store
rows show them in a dedicated meta panel (author, popularity bar, total
downloads, last changed), plus a sort dropdown (most popular / newest / name),
zebra-striped rows, a numbered pager, and a note that the stats refresh daily.
Adds a shared formatRelativeTime helper (formatLastSeen now delegates to it).

* test(themes): cover sort modes, pager jump and relative-time formatting

* fix(build): gate nvidia_quirk_active to Linux

Its only caller is the Linux branch of theme_animation_risk, so on Windows and
macOS the function was dead code and tripped clippy's -D warnings.

* docs: CHANGELOG + credits for theme store download stats & sort (#1036)

src-tauri/src/theme_animation.rs

@@ -15,8 +15,11 @@ pub fn set_nvidia_quirk_active(active: bool) {
     let _ = NVIDIA_QUIRK_ACTIVE.set(active);
 }
 
-/// Whether the Nvidia WebKit quirk was needed at startup. False when unrecorded
-/// (non-Linux, or GPU acceleration opted in via `PSYSONIC_WEBKIT_GPU_ACCEL`).
+/// Whether the Nvidia WebKit quirk was needed at startup. Linux-only: read
+/// solely by the Linux branch of `theme_animation_risk`. False when unrecorded
+/// (GPU acceleration opted in via `PSYSONIC_WEBKIT_GPU_ACCEL`). Gated so it is
+/// not dead code on Windows/macOS, where the quirk never applies.
+#[cfg(target_os = "linux")]
 pub(crate) fn nvidia_quirk_active() -> bool {
     NVIDIA_QUIRK_ACTIVE.get().copied().unwrap_or(false)
 }