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/ScreenRegistry.kt

@@ -14,6 +14,7 @@ import androidx.compose.material.icons.filled.Code
 import androidx.compose.material.icons.filled.Dashboard
 import androidx.compose.material.icons.filled.EmojiEvents
 import androidx.compose.material.icons.filled.Extension
+import androidx.compose.material.icons.filled.Folder
 import androidx.compose.material.icons.filled.History
 import androidx.compose.material.icons.filled.HistoryEdu
 import androidx.compose.material.icons.filled.Info
@@ -33,6 +34,7 @@ import com.m57.hermescontrol.ui.channels.ChannelsScreen as ChannelsScreenContent
 import com.m57.hermescontrol.ui.chat.ChatScreen as ChatScreenContent
 import com.m57.hermescontrol.ui.config.ConfigScreen as ConfigScreenContent
 import com.m57.hermescontrol.ui.cron.CronJobsScreen as CronJobsScreenContent
+import com.m57.hermescontrol.ui.files.FilesScreen as FilesScreenContent
 import com.m57.hermescontrol.ui.gateway.GatewayScreen as GatewayScreenContent
 import com.m57.hermescontrol.ui.kanban.KanbanScreen as KanbanScreenContent
 import com.m57.hermescontrol.ui.keys.KeysScreen as KeysScreenContent
@@ -196,6 +198,12 @@ object ScreenRegistry {
                 Icons.Filled.AccountBalanceWallet,
                 DrawerSection.INSPECT,
             ) { sessionId, openDrawer -> BillingScreenContent(onOpenDrawer = openDrawer) },
+            ScreenDefinition(
+                FilesScreen,
+                R.string.screen_files,
+                Icons.Filled.Folder,
+                DrawerSection.INSPECT,
+            ) { sessionId, openDrawer -> FilesScreenContent(onOpenDrawer = openDrawer) },
             ScreenDefinition(
                 AchievementsScreen,
                 R.string.screen_achievements,