feat: add Files tab backed by the managed-files endpoint (#727)

* feat: add Files tab backed by the managed-files endpoint

Add a Hermes Mobile tab that browses the gateway's managed-files root via the
existing /api/files* backend routes: list/navigate folders, breadcrumb path,
create folder, upload (streaming multipart), delete, and open files via the
authenticated GatewayFileClient download URL.

- data/model/ManagedFile: entry + list/read/upload/mkdir/delete models
- HermesApiService: list/read/upload(+stream)/mkdir/delete endpoints
- FilesViewModel: state, navigation, per-row busy, transient-state cleanup
- FilesScreen: folder list, breadcrumb, FAB new folder, upload picker,
  delete confirm, open via system viewer
- NavigationKeys + ScreenRegistry + strings: register in Inspect drawer section

* fix(files): open files in-app via FileProvider instead of browser URL

The previous file-open path handed a ?token=-authenticated URL to the system
browser, but /api/files/download only accepts the ephemeral in-process session
token as ?token=, not the mobile's Bearer token — so the request failed and the
browser rendered the JSON error body instead of the file.

Fix: download the file in-app through the authenticated client (Bearer/
session-cookie, same path as the rest of the app), decode the base64 data_url,
write it to cacheDir, and open it via FileProvider.getUriForFile + ACTION_VIEW
(mirrors the existing ChatScreen/SystemViewModel open-from-cache pattern).
Works on a remote phone since the file is materialized locally first.

---------

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/NavigationKeys.kt

@@ -53,6 +53,8 @@ import kotlinx.serialization.Serializable
 
 @Serializable data object BillingScreen : NavKey
 
+@Serializable data object FilesScreen : NavKey
+
 // ── Settings drill-down sub-pages ──────────────────────────────────────
 
 @Serializable data object SettingsConnection : NavKey