feat: lyrics provider extensions, configurable lyrics cascade, and iOS method channel parity

Add lyrics_provider as a new extension type alongside metadata_provider and
download_provider. Extensions implementing fetchLyrics() are called before
built-in providers, giving user-installed extensions highest priority.

Built-in lyrics cascade is now configurable from Download Settings:
  - Reorderable provider list (LRCLIB, Musixmatch, Netease, Apple Music, QQ Music)
  - Per-provider options: Netease translation/romanization, Apple/QQ multi-person
    word-by-word speaker tags, Musixmatch language code
  - Provider order and options synced to Go backend on app start and on change

Go backend changes:
  - New lyrics_provider manifest type with validation (extension_manifest.go)
  - ExtensionProviderWrapper.FetchLyrics() with Goja JS bridge (extension_providers.go)
  - Configurable SetLyricsProviderOrder/GetLyricsProviderOrder cascade (lyrics.go)
  - LyricsFetchOptions struct for per-provider settings (lyrics.go)
  - Extracted tryLRCLIB() helper, randomized LRCLIB User-Agent (lyrics.go)
  - Refactored msToLRCTimestamp to separate msToLRCTimestampInline (lyrics.go)
  - New provider source files: lyrics_apple.go, lyrics_musixmatch.go,
    lyrics_netease.go, lyrics_qqmusic.go
  - JSON export functions for lyrics settings (exports.go)
  - hasLyricsProvider field in extension manager JSON output

Platform channels:
  - Android (MainActivity.kt): setLyricsProviders, getLyricsProviders,
    getAvailableLyricsProviders, setLyricsFetchOptions, getLyricsFetchOptions
  - iOS (AppDelegate.swift): same 5 method channel handlers for iOS parity

Flutter side:
  - Extension model: hasLyricsProvider field + Lyrics Provider capability badge
  - Settings model: lyricsProviders, lyricsIncludeTranslationNetease,
    lyricsIncludeRomanizationNetease, lyricsMultiPersonWordByWord,
    musixmatchLanguage fields with generated serialization
  - Settings provider: setters + _syncLyricsSettingsToBackend()
  - Download settings UI: provider picker, toggle switches, language picker
  - Platform bridge: lyrics provider/options methods

Docs: lyrics provider extension documentation in site/docs.html
CHANGELOG: updated with lyrics provider and search feature entries

android/app/src/main/kotlin/com/zarz/spotiflac/MainActivity.kt

@@ -1756,6 +1756,60 @@ class MainActivity: FlutterFragmentActivity() {
                             }
                             result.success(response)
                         }
+                        "setLyricsProviders" -> {
+                            val providersJson = call.argument<String>("providers_json") ?: "[]"
+                            val response = withContext(Dispatchers.IO) {
+                                try {
+                                    Gobackend.setLyricsProvidersJSON(providersJson)
+                                    """{"success":true}"""
+                                } catch (e: Exception) {
+                                    """{"success":false,"error":"${e.message?.replace("\"", "'")}"}"""
+                                }
+                            }
+                            result.success(response)
+                        }
+                        "getLyricsProviders" -> {
+                            val response = withContext(Dispatchers.IO) {
+                                try {
+                                    Gobackend.getLyricsProvidersJSON()
+                                } catch (e: Exception) {
+                                    "[]"
+                                }
+                            }
+                            result.success(response)
+                        }
+                        "getAvailableLyricsProviders" -> {
+                            val response = withContext(Dispatchers.IO) {
+                                try {
+                                    Gobackend.getAvailableLyricsProvidersJSON()
+                                } catch (e: Exception) {
+                                    "[]"
+                                }
+                            }
+                            result.success(response)
+                        }
+                        "setLyricsFetchOptions" -> {
+                            val optionsJson = call.argument<String>("options_json") ?: "{}"
+                            val response = withContext(Dispatchers.IO) {
+                                try {
+                                    Gobackend.setLyricsFetchOptionsJSON(optionsJson)
+                                    """{"success":true}"""
+                                } catch (e: Exception) {
+                                    """{"success":false,"error":"${e.message?.replace("\"", "'")}"}"""
+                                }
+                            }
+                            result.success(response)
+                        }
+                        "getLyricsFetchOptions" -> {
+                            val response = withContext(Dispatchers.IO) {
+                                try {
+                                    Gobackend.getLyricsFetchOptionsJSON()
+                                } catch (e: Exception) {
+                                    "{}"
+                                }
+                            }
+                            result.success(response)
+                        }
                         "reEnrichFile" -> {
                             val requestJson = call.argument<String>("request_json") ?: "{}"
                             val response = withContext(Dispatchers.IO) {