Rename abbreviated identifiers in hiddify core service layer for readability

[Copilot]

Cryptic single/two-letter names in the core gRPC service layer required deep context to understand. Replaces them with self-documenting names throughout CoreInterface, its subclasses, and HiddifyCoreService.

Renamed identifiers

CoreInterface fields & methods (propagated to all subclasses and callers):

• fgClient / bgClient → foregroundClient / backgroundClient
• isSingleChannel() → isUsingSharedChannel()
• isActiveFg() / isActiveBg() → isForegroundServiceActive() / isBackgroundServiceActive()
• isBgClientAvailable() → isBackgroundClientAvailable()

HiddifyCoreService internals:

• listenSingle() / stopListenSingle() → registerSingleStreamListener() / cancelStreamListenerByKey()
• Parameters: cc → coreClient, key → listenerKey, stream → streamFactory
• Local vars: errMsg → errorMessage, sub → subscription
• Generic res → purpose-specific: settingsResponse, startResponse, restartResponse, selectResponse, urlTestResponse, helloResponse, tunnelResetSucceeded

Example

// Before
await startListeningLogs("fg", core.fgClient);
if (!core.isSingleChannel()) {
  await startListeningLogs("bg", core.bgClient);
}

// After
await startListeningLogs("fg", core.foregroundClient);
if (!core.isUsingSharedChannel()) {
  await startListeningLogs("bg", core.backgroundClient);
}

Pure rename — no behavioral changes.

Original prompt

Pull Request: https://github.com/iAlien369-design/hiddify-app/pull/2/files

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