test(mobile): the gate checks the File Viewer button's phone default again (#809)

Until #809 a display:none rule in mobile.css and its KNOWN_PHONE_HIDDEN
entry kept the File Viewer button off phones, and the gate checked both.
With that rule gone, the handheld default in getDefaultSettings() and the
html.mobile-init startup rule are the whole lock, and only the browser
test saw them, which npm test does not run.

The policy test now loads the real getDefaultSettings() on a handheld and
on a desktop, checks that every reader of the setting goes through the
device default, and that mobile.css keeps the startup hide.

Docs: architecture-invariants, src/web/CLAUDE.md and the wiki say the
button is off on a phone until it is switched on there, and that a window
narrower than 600px on a device that is not a handheld now shows it.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

docs/wiki/Mobile-Guide.md

@@ -191,12 +191,14 @@ last answer as scrollable text instead, with a **More** button for additional co
 Persian, Arabic and Hebrew answers read right to left there.
 
 The [File Viewer](Working-With-Files) works on phones too, including edit mode, which is
-enough to fix a typo an agent introduced while you are away from your desk.
+enough to fix a typo an agent introduced while you are away from your desk. Its header button
+is off on a phone until you turn on **File Viewer** under **App Settings → Header & Panels**
+on that phone.
 
 ## What is deliberately not on phones
 
 - Extra header buttons. New header controls are kept off phones by policy, with a test that
-  enforces it.
+  enforces it. The one exception is File Viewer, which you can switch on (see above).
 - The Approvals bell. Phones get the NEEDS YOU strips on the home screen instead.
 - The desktop home tab rail, which needs a wide window.
 - Lineage lines, which are a desktop overlay.

docs/wiki/The-Dashboard.md

@@ -154,7 +154,7 @@ The right side of the header. Almost all of these are off until you enable them
 | Connection dot         | Always on          | SSE connection health. Green is connected.                                       |
 | Font size `-` / `+`    | Always on          | `Ctrl +` / `Ctrl -` do the same.                                                 |
 | CPU / MEM              | On                 | Server resource use. Drawn as a compact pill by default; see Header Stats Style below. |
-| File Viewer            | On                 | Toggles the file browser panel.                                                  |
+| File Viewer            | On, off on phones  | Toggles the file browser panel.                                                  |
 | Settings gear          | Always on          | App Settings.                                                                    |
 | Plan usage chip        | On, desktop only   | Live subscription usage: Claude (needs its telemetry exporter, which the same setting installs), Codex limits and the Copilot monthly quota. |
 | Session Manager        | Off                | The full session list, live and historical.                                      |

docs/wiki/Working-With-Files.md

@@ -8,7 +8,8 @@ until you get home.
 
 A panel that browses the active session's working directory, or the git worktree its agent
 moved into ([Git Worktrees](Git-Worktrees#when-an-agent-moves-into-a-worktree)). Its header
-button is on by default; if it is missing, re-enable it in **App Settings → Header & Panels**.
+button is on by default on a desktop and off on a phone. The setting is per device: switch it
+in **App Settings → Header & Panels** on the device you want it on.
 
 The panel and its search list up to 5,000 files and folders per load, shallow levels first and
 taking turns between folders, so one huge folder cannot use up that limit before the ones after it,