refactor(ui): R10 S4 — complete the selection-paint sweep (all pill idioms)

The first S4 pass migrated a subset, which left same-idiom selection pills
hand-rolled and made adjacent controls inconsistent (a /code-review finding).
This completes the sweep so every selected-among-options control shares the
recipe:

- plain pills → .pill (+ aria-pressed): Channels status filters + view tabs,
  Sidebar filter-tag chips + date presets + custom-dates, Stats tabs + ranges.
- glass-card-resting pills → .pill with the resting `glass-card glass-hover` kept
  in the non-pressed branch (solid accent when pressed): PlexSidebar library
  scope + the facet Chip (retired the round-1 "deferred" note — the recipe
  already expresses this via the resting branch, no new variant needed).
- custom-dates aria-pressed is coerced with !! so a date string can't leak into
  the attribute and miss the [aria-pressed="true"] paint.
- drop the dead `border` on ChannelTags' chip-solid (accent border on accent bg).

Out of scope (a separate concern, NOT selection-among-options): primary/action
buttons that share the accent fill — Banner action, ConfirmProvider primary,
PlexBrowse primary, PlayerModal watched-toggle — these want a future .btn-primary
recipe. Left by-design: tableHeader filter options (PopoverItem + keepSelectedPaint,
owned by the popover platform).

frontend/src/components/Stats.tsx

@@ -35,11 +35,8 @@ export default function Stats() {
             <button
               key={id}
               onClick={() => setTab(id)}
-              className={`px-3 py-1.5 rounded-full text-sm border transition ${
-                tab === id
-                  ? "bg-accent text-accent-fg border-accent"
-                  : "bg-card border-border text-muted hover:border-accent"
-              }`}
+              aria-pressed={tab === id}
+              className="pill px-3 py-1.5 rounded-full text-sm border"
             >
               {t(`stats.tabs.${id}`)}
             </button>
@@ -236,11 +233,8 @@ function SystemStats() {
             <button
               key={d}
               onClick={() => setDays(d)}
-              className={`px-3 py-1.5 rounded-full text-sm border transition ${
-                days === d
-                  ? "bg-accent text-accent-fg border-accent"
-                  : "bg-card border-border text-muted hover:border-accent"
-              }`}
+              aria-pressed={days === d}
+              className="pill px-3 py-1.5 rounded-full text-sm border"
             >
               {t("stats.rangeDays", { count: d })}
             </button>