docs(platform.frontend-template): filter header does not recolor on active filters

Records the template change: the Filters sidebar header no longer tints blue
when filters are active — the count pill + active-filter chips signal it. Aligns
with devices/fleet (which never recolored); the old behaviour was the outlier.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/main/resources/content/platform/frontend-template/rules.md

@@ -74,3 +74,10 @@ content frame, never in `.header`. Add/Edit forms are **slide-in panels**, not c
 Locked structure: `.filter-sidebar` · `.content-frame` · `.table-card` border · active-filter chips ·
 `Pagination` at top-of-table. New page → **copy** TablePage/TemplatePage first; existing page → verify
 it conforms before building on it (many drifted).
+
+**The Filters sidebar header does NOT recolor when filters are active.** The active state is signalled
+by the count pill and the removable active-filter chips above the table — not by tinting the header
+bar blue. (The `bar-filters-active` class binding is kept for structural parity but its CSS is a
+no-op.) This aligns the template with the devices and fleet SPAs, which never recolored the header;
+the old blue-header behaviour in the template/incident was the outlier. Do not re-add the header
+tint when copying a page.