fix(ui): keep one Back arrow on detail pages (#1627)

Movie and series detail pages showed two arrow_back controls while the
inline player was open: the shell's sticky arrow (added in #1576 so Back
survives scrolling) meant "Close player" in watch state, while the
now-playing bar carried a second arrow that meant route-level Back — the
same icon with two meanings, next to a "Close player" button that
duplicated the first.

The shell's sticky arrow is now route-level Back in browse and watch
alike, and the bar carries no arrow of its own. Closing the player is
the bar's "Close player" button and Escape, which still unwinds one
level (close, then back). Hosts without a browse Back target (M3U,
downloads) render no arrow in either state.

Unit specs for the shell and the inline player cover the new contract;
the Electron and web E2E helpers that pressed Back from watch are
updated, and the M3U flow closes the player through the bar's button.
Docs, the mirrored CLAUDE.md/AGENTS.md paragraph and a release note
follow the change.

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

apps/electron-backend-e2e/src/catalog-sorting.e2e.ts

@@ -590,8 +590,8 @@ async function firstVisibleGridTitle(page: Page): Promise<string> {
 }
 
 async function goBackFromDetail(page: Page): Promise<void> {
-    // Return to the list: browse uses the sticky Back, watch uses the
-    // now-playing bar's direct Back (the sticky watch action is Close player).
+    // Return to the list: the shell's sticky Back is route-level in browse
+    // and watch alike (closing the player is the bar's own Close button).
     const backButton = page
         .locator('app-portal-detail-shell')
         .first()

apps/electron-backend-e2e/src/dashboard-activation.e2e.ts

@@ -249,8 +249,8 @@ function dashboardRailCardByTitle(
 }
 
 async function goBackFromDetail(page: Page): Promise<void> {
-    // Return to the list: browse uses the sticky Back, watch uses the
-    // now-playing bar's direct Back (the sticky watch action is Close player).
+    // Return to the list: the shell's sticky Back is route-level in browse
+    // and watch alike (closing the player is the bar's own Close button).
     const backButton = page
         .locator('app-portal-detail-shell')
         .first()

apps/electron-backend-e2e/src/favorites.e2e.ts

@@ -606,8 +606,8 @@ async function addCurrentDetailToFavorites(page: Page): Promise<void> {
 }
 
 async function goBackFromDetail(page: Page): Promise<void> {
-    // Return to the list: browse uses the sticky Back, watch uses the
-    // now-playing bar's direct Back (the sticky watch action is Close player).
+    // Return to the list: the shell's sticky Back is route-level in browse
+    // and watch alike (closing the player is the bar's own Close button).
     const backButton = page
         .locator('app-portal-detail-shell')
         .first()

apps/electron-backend-e2e/src/recent.e2e.ts

@@ -653,8 +653,8 @@ async function expectUnifiedLiveDetailOpen(
 }
 
 async function goBackFromDetail(page: Page): Promise<void> {
-    // Return to the list: browse uses the sticky Back, watch uses the
-    // now-playing bar's direct Back (the sticky watch action is Close player).
+    // Return to the list: the shell's sticky Back is route-level in browse
+    // and watch alike (closing the player is the bar's own Close button).
     const backButton = page
         .locator('app-portal-detail-shell')
         .first()

apps/electron-backend-e2e/src/search.e2e.ts

@@ -1540,8 +1540,8 @@ async function addCurrentDetailToFavorites(page: Page): Promise<void> {
 }
 
 async function goBackFromDetail(page: Page): Promise<void> {
-    // Return to the list: browse uses the sticky Back, watch uses the
-    // now-playing bar's direct Back (the sticky watch action is Close player).
+    // Return to the list: the shell's sticky Back is route-level in browse
+    // and watch alike (closing the player is the bar's own Close button).
     const backButton = page
         .locator('app-portal-detail-shell')
         .first()