Remove embedded music player

The music feature is retired: delete the Media tab, MediaAPI,
MediaPlayerController, PinnedMediaResourceLoader, MediaPanelView, the
media_state control publish, media scope handling in VoiceSession, the
volume setting, and all media tests. App is now Talk + Activity tabs.

Backend media API in hope-voice-api is untouched (still serves the web UI).

UITests/VoiceUITests.swift

@@ -48,29 +48,6 @@ final class VoiceUITests: XCTestCase {
         XCTAssertEqual(app.textFields["Server URL"].value as? String, "https://127.0.0.1:1")
     }
 
-    func testMusicSearchAndStationValidation() {
-        app.launch()
-        app.tabBars.buttons["Music"].tap()
-        XCTAssertTrue(app.buttons["Add radio station"].waitForExistence(timeout: 5))
-        XCTAssertTrue(app.staticTexts["Music server unavailable"].waitForExistence(timeout: 10))
-        app.segmentedControls.buttons["Search"].tap()
-        let query = app.textFields["Song, artist, lyrics…"]
-        query.tap()
-        query.typeText("Piano")
-        app.buttons["music.search"].tap()
-        XCTAssertTrue(app.staticTexts.matching(NSPredicate(format: "label CONTAINS 'connect to the server'")).firstMatch.waitForExistence(timeout: 15))
-        app.buttons["Add radio station"].tap()
-        let url = app.textFields["Audio stream URL"]
-        XCTAssertTrue(url.waitForExistence(timeout: 5))
-        url.tap()
-        url.typeText("not-a-url")
-        app.buttons["Add station"].tap()
-        XCTAssertTrue(app.staticTexts["Enter a complete http:// or https:// audio stream address."].exists)
-        app.buttons["Cancel"].tap()
-        XCTAssertTrue(app.buttons["Add radio station"].exists)
-        capture("Music search")
-    }
-
     func testTranscriptClearConfirmationAndActivityDetails() {
         app.launchArguments.append("--ui-fixtures")
         app.launch()