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/test/java/com/m57/hermescontrol/data/remote/GatewayFileClientTest.kt

@@ -0,0 +1,78 @@
+package com.m57.hermescontrol.data.remote
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class GatewayFileClientTest {
+    private val base = "https://gw.example.com:9119"
+    private val tok = "s e/cret"
+
+    @Test
+    fun `buildDownloadUrl encodes path and token`() {
+        val url = GatewayFileClient.buildDownloadUrl(base, tok, "/tmp/foo.png")!!
+        assertEquals(
+            "$base/api/files/download?path=%2Ftmp%2Ffoo.png&token=${java.net.URLEncoder.encode(
+                tok,
+                "UTF-8",
+            ).replace("+", "%20")}",
+            url,
+        )
+        assertTrue(url.contains("path=%2Ftmp%2Ffoo.png"))
+        assertTrue(url.contains("token="))
+    }
+
+    @Test
+    fun `buildDownloadUrl rejects blank base or token`() {
+        assertNull(GatewayFileClient.buildDownloadUrl("", tok, "/tmp/x.png"))
+        assertNull(GatewayFileClient.buildDownloadUrl(base, "", "/tmp/x.png"))
+    }
+
+    @Test
+    fun `buildDownloadUrl rejects relative paths`() {
+        assertNull(GatewayFileClient.buildDownloadUrl(base, tok, "relative/path.png"))
+        assertNull(GatewayFileClient.buildDownloadUrl(base, tok, "MEDIA:relative.png"))
+    }
+
+    @Test
+    fun `buildDownloadUrl handles quoted and spaced paths`() {
+        val url = GatewayFileClient.buildDownloadUrl(base, tok, "\"/tmp/a b.png\"")!!
+        assertTrue(url.contains("path=%2Ftmp%2Fa%20b.png"))
+        assertFalse(url.contains("MEDIA:"))
+    }
+
+    @Test
+    fun `normalizePath expands tilde`() {
+        val home = System.getenv("HOME") ?: "/home/test"
+        assertEquals("$home/foo.png", GatewayFileClient.normalizePath("~/foo.png"))
+    }
+
+    @Test
+    fun `normalizePath strips surrounding quotes`() {
+        assertEquals("/tmp/x.png", GatewayFileClient.normalizePath("'/tmp/x.png'"))
+        assertEquals("/tmp/x.png", GatewayFileClient.normalizePath("`/tmp/x.png`"))
+    }
+
+    @Test
+    fun `normalizePath requires absolute path`() {
+        assertNull(GatewayFileClient.normalizePath("relative.png"))
+    }
+
+    @Test
+    fun `classifyStatus maps known codes`() {
+        assertEquals(GatewayFileResult.NotFound, GatewayFileClient.classifyStatus(404))
+        assertEquals(GatewayFileResult.Forbidden, GatewayFileClient.classifyStatus(403))
+        assertEquals(GatewayFileResult.TooLarge, GatewayFileClient.classifyStatus(413))
+        assertEquals(GatewayFileResult.Unauthorized, GatewayFileClient.classifyStatus(401))
+        assertNull(GatewayFileClient.classifyStatus(200))
+        assertNull(GatewayFileClient.classifyStatus(500))
+    }
+
+    @Test
+    fun `parseFilename extracts from content-disposition`() {
+        assertEquals("report.pdf", GatewayFileClient.parseFilename("attachment; filename=\"report.pdf\""))
+        assertEquals("a b.png", GatewayFileClient.parseFilename("inline; filename*=UTF-8''a%20b.png"))
+    }
+}