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/data/model/ManagedFile.kt

@@ -0,0 +1,96 @@
+package com.m57.hermescontrol.data.model
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+/**
+ * A single entry returned by `GET /api/files` — a file or directory living
+ * under the gateway's managed-files root.
+ *
+ * Mirrors the backend `_managed_file_entry()` dict exactly
+ * (hermes_cli/web_server.py). `mtime` is an epoch float on the wire; the
+ * mobile model declares it `Double?` so a fractional value does not throw a
+ * kotlinx decoding error (see dashboard-tab-to-android pitfalls: numeric
+ * type drift).
+ */
+@Serializable
+data class ManagedFileEntry(
+    val name: String = "",
+    val path: String = "",
+    @SerialName("is_directory")
+    val isDirectory: Boolean = false,
+    val size: Long? = null,
+    val mtime: Double? = null,
+    @SerialName("mime_type")
+    val mimeType: String? = null,
+)
+
+/**
+ * Response of `GET /api/files` (directory listing with navigation metadata).
+ *
+ * `parent` is `null` at the managed root (or when a locked root hides the
+ * parent). `entries` is already server-sorted directories-first, then
+ * case-insensitive name.
+ */
+@Serializable
+data class ManagedFilesListResponse(
+    val path: String = "",
+    val parent: String? = null,
+    val entries: List<ManagedFileEntry> = emptyList(),
+    val root: String? = null,
+    @SerialName("locked_root")
+    val lockedRoot: String? = null,
+    @SerialName("can_change_path")
+    val canChangePath: Boolean = false,
+)
+
+/** Response of `GET /api/files/read` — base64 data URL of a file's bytes. */
+@Serializable
+data class ManagedFileRead(
+    val name: String = "",
+    val path: String = "",
+    val size: Long = 0,
+    @SerialName("mime_type")
+    val mimeType: String = "application/octet-stream",
+    @SerialName("data_url")
+    val dataUrl: String = "",
+    val root: String? = null,
+    @SerialName("locked_root")
+    val lockedRoot: String? = null,
+    @SerialName("can_change_path")
+    val canChangePath: Boolean = false,
+)
+
+/** Generic `{"ok": true}`-style acknowledgement used by upload/mkdir/delete. */
+@Serializable
+data class ManagedFileActionResponse(
+    val ok: Boolean = false,
+    val path: String? = null,
+    val entry: ManagedFileEntry? = null,
+    val root: String? = null,
+    @SerialName("locked_root")
+    val lockedRoot: String? = null,
+    @SerialName("can_change_path")
+    val canChangePath: Boolean = false,
+)
+
+/** Request body for `POST /api/files/upload` (base64 data URL form). */
+@Serializable
+data class ManagedFileUpload(
+    val path: String,
+    val data_url: String,
+    val overwrite: Boolean = false,
+)
+
+/** Request body for `POST /api/files/mkdir`. */
+@Serializable
+data class ManagedDirectoryCreate(
+    val path: String,
+)
+
+/** Request body for `DELETE /api/files`. */
+@Serializable
+data class ManagedFileDelete(
+    val path: String,
+    val recursive: Boolean = false,
+)