feat: gateway files client — fetch any gateway-hosted file over HTTP (#726)

* feat: gateway files client — fetch any gateway-hosted file over HTTP

Add GatewayFileClient: a typed client for the gateway's managed-files
download endpoint (GET /api/files/download?path=<enc>&token=<enc>), which
streams the raw bytes of any file living on the gateway host. This is the
mobile equivalent of desktop's mediaExternalUrl() and works on a remote
phone (real HTTP, not a host-local file read).

- buildDownloadUrl(): pure authed-URL builder (null on blank creds /
  non-absolute path), mirrors desktop's token-in-query auth.
- fetch(): OkHttp GET via the shared client, maps 401/403/404/413 to typed
  GatewayFileResult variants; returns GatewayFile(name, mime, bytes).
- MediaTypes: IMAGE/AUDIO/VIDEO/FILE classification by extension (mirrors
  desktop MEDIA_BY_EXT) so future consumers can present images, audio,
  video, or generic files — beyond just images.

Pure builders/classifiers are unit-tested. Mobile-only, backend untouched.
This is the foundation PR; issue #724's MEDIA: rendering will consume it.

* fix: decode filename + handle download-URL in mediaNameFromPath

- GatewayFileClient.parseFilename now URL-decodes the captured value so
  filename*=UTF-8''a%20b.png yields 'a b.png'.
- MediaTypes.mediaNameFromPath extracts the filename from the percent-encoded
  path= query param of a full /api/files/download?path=<enc> URL (the real
  case), not just a bare path.

* fix(remote): fix unit tests for gateway file client and media parsing

Replace plus-encoded spaces with %20 in download URL builder, URL-decode filenames in parseFilename, and extract target filename from URL path parameters.

---------

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

app/src/main/java/com/m57/hermescontrol/ui/chat/MediaTypes.kt

@@ -0,0 +1,93 @@
+package com.m57.hermescontrol.ui.chat
+
+import java.net.URLDecoder
+import java.nio.charset.StandardCharsets
+
+/**
+ * File-kind classification for gateway-hosted files, mirroring the desktop app's
+ * `MEDIA_BY_EXT` table (`apps/desktop/src/lib/media.ts`). Lets UI code decide
+ * how to present a downloaded file — inline image, audio player, video player,
+ * or a generic file card — without re-deriving the mapping at every call site.
+ *
+ * Extend the table (not the call sites) when new extensions are needed.
+ */
+enum class MediaKind {
+    IMAGE,
+    AUDIO,
+    VIDEO,
+    FILE,
+}
+
+private val MEDIA_BY_EXT: Map<String, Pair<MediaKind, String>> =
+    mapOf(
+        "avi" to (MediaKind.VIDEO to "video/x-msvideo"),
+        "bmp" to (MediaKind.IMAGE to "image/bmp"),
+        "csv" to (MediaKind.FILE to "text/csv"),
+        "flac" to (MediaKind.AUDIO to "audio/flac"),
+        "gif" to (MediaKind.IMAGE to "image/gif"),
+        "jpeg" to (MediaKind.IMAGE to "image/jpeg"),
+        "jpg" to (MediaKind.IMAGE to "image/jpeg"),
+        "m4a" to (MediaKind.AUDIO to "audio/mp4"),
+        "mkv" to (MediaKind.VIDEO to "video/x-matroska"),
+        "mov" to (MediaKind.VIDEO to "video/quicktime"),
+        "mp3" to (MediaKind.AUDIO to "audio/mpeg"),
+        "mp4" to (MediaKind.VIDEO to "video/mp4"),
+        "ogg" to (MediaKind.AUDIO to "audio/ogg"),
+        "opus" to (MediaKind.AUDIO to "audio/ogg; codecs=opus"),
+        "pdf" to (MediaKind.FILE to "application/pdf"),
+        "png" to (MediaKind.IMAGE to "image/png"),
+        "svg" to (MediaKind.IMAGE to "image/svg+xml"),
+        "wav" to (MediaKind.AUDIO to "audio/wav"),
+        "webm" to (MediaKind.VIDEO to "video/webm"),
+        "webp" to (MediaKind.IMAGE to "image/webp"),
+    )
+
+/** Classify a path/URL by extension. Unknown extensions fall back to [MediaKind.FILE]. */
+fun mediaKindForPath(path: String): MediaKind {
+    val ext =
+        path
+            .split('?', limit = 2)[0]
+            .split('.')
+            .lastOrNull()
+            ?.lowercase()
+            .orEmpty()
+    return MEDIA_BY_EXT[ext]?.first ?: MediaKind.FILE
+}
+
+/** Best-guess MIME type for a path/URL by extension. Falls back to octet-stream. */
+fun mediaMimeForPath(path: String): String {
+    val ext =
+        path
+            .split('?', limit = 2)[0]
+            .split('.')
+            .lastOrNull()
+            ?.lowercase()
+            .orEmpty()
+    return MEDIA_BY_EXT[ext]?.second ?: "application/octet-stream"
+}
+
+/** Trailing filename from a path or URL.
+ *
+ * Handles both a bare gateway path (`/tmp/report.pdf`) and a full
+ * `/api/files/download?path=<enc>` URL, where the real filename lives in the
+ * percent-encoded `path=` query parameter. */
+fun mediaNameFromPath(path: String): String {
+    val queryPathMatch = Regex("""[?&]path=([^&]+)""", RegexOption.IGNORE_CASE).find(path)
+    if (queryPathMatch != null) {
+        val targetPath =
+            runCatching {
+                URLDecoder.decode(queryPathMatch.groupValues[1], StandardCharsets.UTF_8.name())
+            }.getOrDefault(queryPathMatch.groupValues[1])
+        targetPath.split('/', '\\').lastOrNull { it.isNotBlank() }?.let { return it }
+    }
+    val cleanPath = path.split('?', limit = 2)[0]
+    return runCatching {
+        java.net
+            .URI(cleanPath)
+            .path
+            .split('/')
+            .lastOrNull { it.isNotBlank() }
+    }.getOrNull()
+        ?: cleanPath.split('/', '\\').lastOrNull { it.isNotBlank() }
+        ?: path
+}