refactor: standardize constant naming and improve file-based auth handling

- Renamed constants from uppercase to CamelCase for consistency.
- Replaced redundant file-based auth handling logic with the new `util.CountAuthFiles` helper.
- Fixed various error-handling inconsistencies and enhanced robustness in file operations.
- Streamlined auth client reload logic in server and watcher components.
- Applied minor code readability improvements across multiple packages.

internal/client/gemini-web/request.go

@@ -79,10 +79,6 @@ func SendWithSplit(chat *ChatSession, text string, files []string, cfg *config.C
 		useHint = false
 		chunkSize = maxChars
 	}
-	if chunkSize <= 0 {
-		// As a last resort, split by single rune to avoid exceeding the limit
-		chunkSize = 1
-	}
 
 	// Split into rune-safe chunks
 	chunks := ChunkByRunes(text, chunkSize)