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/web-e2e/src/m3u-movie-details.e2e.ts

@@ -406,11 +406,13 @@ test('@web @m3u @tmdb browse and watch keep the adjusted volume', async ({
             )
         )
         .toBe(0.25);
-    // M3U has no browse Back target, but the sticky watch control can close it.
+    // M3U has no browse Back target, so the shell shows no arrow in either
+    // state; the now-playing bar's own Close button returns to browse.
     const shell = detail(page).locator('app-portal-detail-shell');
+    await expect(shell.locator('.shell__back-button')).toHaveCount(0);
     await shell
+        .locator('app-portal-inline-player')
         .getByRole('button', { name: 'Close player', exact: true })
-        .first()
         .click();
     await expect(inlineVideo(page)).toHaveCount(0);
     await expect(shell.locator('.shell__back-button')).toHaveCount(0);