refactor(gemini-web): Remove auto-refresh, auto-close, and caching

This commit simplifies the Gemini web client by removing several complex, stateful features. The previous implementation for auto-refreshing cookies and auto-closing the client involved background goroutines, timers, and file system caching, which made the client's lifecycle difficult to manage.

The following features have been removed:
- The cookie auto-refresh mechanism, including the background goroutine (`rotateCookies`) and related configuration fields.
- The file-based caching for the `__Secure-1PSIDTS` token. The `rotate1PSIDTS` function now fetches a new token on every call.
- The auto-close functionality, which used timers to close the client after a period of inactivity.
- Associated configuration options and methods (`WithAccountLabel`, `WithOnCookiesRefreshed`, `Close`, etc.).

By removing this logic, the client becomes more stateless and predictable. The responsibility for managing the client's lifecycle and handling token expiration is now shifted to the caller, leading to a simpler and more robust integration.

internal/watcher/watcher.go

@@ -461,9 +461,6 @@ func (w *Watcher) reloadConfig() bool {
 		if oldConfig.GeminiWeb.DisableContinuationHint != newConfig.GeminiWeb.DisableContinuationHint {
 			log.Debugf("  gemini-web.disable-continuation-hint: %t -> %t", oldConfig.GeminiWeb.DisableContinuationHint, newConfig.GeminiWeb.DisableContinuationHint)
 		}
-		if oldConfig.GeminiWeb.TokenRefreshSeconds != newConfig.GeminiWeb.TokenRefreshSeconds {
-			log.Debugf("  gemini-web.token-refresh-seconds: %d -> %d", oldConfig.GeminiWeb.TokenRefreshSeconds, newConfig.GeminiWeb.TokenRefreshSeconds)
-		}
 		if oldConfig.GeminiWeb.CodeMode != newConfig.GeminiWeb.CodeMode {
 			log.Debugf("  gemini-web.code-mode: %t -> %t", oldConfig.GeminiWeb.CodeMode, newConfig.GeminiWeb.CodeMode)
 		}