feat(workspace): route platform Back gestures to the header Back (#1943)

* feat(workspace): route platform Back gestures to the header Back

The mouse Back button, Alt+Left (Cmd+[ on macOS) and, in Electron, the
window's app-command browser-backward (Windows/Linux) and trackpad swipe
(macOS) now run the workspace header's Back, one step each.

- WorkspaceBackGesturesService (shell-provided) dispatches every gesture
  through WorkspaceBackNavigationService.goBack(). A gesture is swallowed
  while a dialog, menu, the phone drawer or a fullscreen player is up, and
  left to the browser only where the header has no Back.
- The chord is a navigate-back entry in the shortcuts dialog, handled in a
  capture-phase listener of WorkspaceKeyboardShortcutsService; ignored while
  typing, repeats swallowed.
- Electron forwards app-command and swipe over the push-only channel
  WINDOW:BACK_REQUESTED (preload onWindowBackRequested), gated in the
  renderer on RuntimeCapabilitiesService.supportsWindowBackGestures.
- A mouse press that also arrives as app-command goes back once, however
  long the button is held; Electron's own Chromium history step on the
  mouse button is always prevented.
- No Forward binding.

Docs: workspace-shell (Platform gestures), portal-detail-navigation,
electron-security, UI guidelines, README shortcuts table.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* refactor(workspace): drop NgZone from the Back gestures service

Master runs zoneless change detection and no longer uses NgZone; the
bridge callback calls goBack() directly.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(workspace): keep Back gestures where the Fullscreen API is missing

iPhone Safari has no unprefixed document.fullscreenElement; the
undefined value read as "a player is fullscreen" and blocked every
gesture.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(workspace): keep a held Back chord's repeats swallowed after Back

Repeats asked for the current result, so once the first press reached a
page with no Back left they were released to the browser, which could take
another history step. Repeats now follow the chord's first press until a
key is released.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(electron): one Back per press of the Linux Browser Back key

On Linux Electron raises app-command browser-backward for every
unhandled event of the Browser Back key (NativeWindowViews::
HandleKeyboardEvent), keydown and keyup alike, so one press went back
twice. The main process now consumes that key in before-input-event and
requests Back once per press; mouse Back buttons still arrive as
app-command.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: 4gray <fourgray@proton.me>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

apps/web-e2e/src/workspace-header-back.e2e.ts

@@ -1,4 +1,4 @@
-import type { Page } from '@playwright/test';
+import type { CDPSession, Page } from '@playwright/test';
 import { expect, test } from './fixtures';
 import {
     addXtreamPortal,
@@ -49,6 +49,41 @@ async function openAsFirstEntry(page: Page, url: string): Promise<Page> {
     return firstEntryPage;
 }
 
+/**
+ * The Back chord for the platform the page reports (the web project emulates
+ * Windows): Cmd+[ on macOS, Alt+← elsewhere.
+ */
+async function backChord(page: Page): Promise<string> {
+    const isMac = await page.evaluate(() =>
+        /Mac|iPhone|iPad|iPod/i.test(
+            (navigator as Navigator & { userAgentData?: { platform?: string } })
+                .userAgentData?.platform || navigator.userAgent
+        )
+    );
+    return isMac ? 'Meta+BracketLeft' : 'Alt+ArrowLeft';
+}
+
+/** One mouse Back press through Chromium's input pipeline. */
+async function pressMouseBack(cdp: CDPSession, page: Page): Promise<void> {
+    const box = await page.locator('main').first().boundingBox();
+    const x = Math.round((box?.x ?? 0) + (box?.width ?? 400) / 2);
+    const y = Math.round((box?.y ?? 0) + (box?.height ?? 400) / 2);
+    for (const type of ['mousePressed', 'mouseReleased'] as const) {
+        await cdp.send('Input.dispatchMouseEvent', {
+            type,
+            x,
+            y,
+            button: 'back',
+            buttons: type === 'mousePressed' ? 8 : 0,
+            clickCount: 1,
+        });
+    }
+}
+
+function historyIndex(page: Page): Promise<number> {
+    return page.evaluate(() => window.navigation?.currentEntry?.index ?? -1);
+}
+
 test.beforeEach(async ({ page, request }) => {
     await request.post(`${MOCK_SERVER}/reset`);
     await page.goto('/');
@@ -82,6 +117,35 @@ test('@web @xtream the in-portal search page reached in the app goes back in his
     await page.waitForURL(/\/workspace\/dashboard$/);
 });
 
+test('@web @xtream the mouse Back button and the Back chord go back one step', async ({
+    page,
+}) => {
+    await page.getByRole('link', { name: 'Series', exact: true }).click();
+    await page.waitForURL(/\/series$/);
+    const listUrl = page.url();
+    const listIndex = await historyIndex(page);
+    const cdp = await page.context().newCDPSession(page);
+
+    // The workspace runs Back and keeps the browser from its own step, which
+    // would go back a second time.
+    for (const gesture of [
+        () => pressMouseBack(cdp, page),
+        async () => page.keyboard.press(await backChord(page)),
+    ]) {
+        await page.locator('.category-content-layout mat-card').first().click();
+        await page.waitForURL(/\/series\/[^/]+\/[^/]+$/);
+        await expect(page.locator('app-portal-detail-shell')).toBeVisible();
+
+        await gesture();
+        await page.waitForURL(listUrl);
+        await page.evaluate(
+            () => new Promise((resolve) => setTimeout(resolve, 400))
+        );
+        expect(page.url()).toBe(listUrl);
+        expect(await historyIndex(page)).toBe(listIndex);
+    }
+});
+
 for (const { name, path, selector, parent } of [
     {
         name: 'movie Discover',